Since 1893, there has been an international classification for coding mortality. When the World Health Organization (WHO) was established after the Second World War, it took charge of publishing these classifications. The Sixth Revision of the International Statistical Classification of Diseases, Injuries, and Causes of Death (ICD)  was published in 1948 and soon afterwards it began to to code and tabulate mortality and morbidity data.
In the early years of nomenclature and coding of neoplasms (1950s and 1960s), the principal system for classifying diseases was the ICD series published by WHO. Eventually ICD was used to code and tabulate the diagnoses on medical records for the purpose of storage and retrieval, and Chapter II of ICD was always assigned to neoplasms.
In the Sixth Revision of ICD in 1948, the classification of neoplasms has been based primarily on topographic site and behavior (that is, whether the neoplasm was malignant, benign, or not specified). Except for lymphatic and hematopoietic neoplasms, choriocarcinoma, melanoma, and certain benign neoplasms, there had been no codes assigned for other histologic types.
The first code manual for the morphology of neoplasms was published by the American Cancer Society (ACS) in 1951 as the Manual of Tumor Nomenclature and Coding (MOTNAC) . Tumor codes consisted of a two-digit code for morphology with a third digit denoting the behavior of the neoplasm. This code was the basis of a statistical code proposed by WHO in 1956 for tumor morphology.
In the 1960s, with the aid of the ACS, the College of American Pathologists (CAP) published the Systematized Nomenclature of Pathology (SNOP) . SNOP provided a morphology code including two sections,  and  on neoplasms and a completely new, highly detailed topography code to cover the whole body. An agreement stipulated that ACS could use the SNOP neoplasm morphology sections 8 and 9 and publish these with their own topography codes. Since cancer registries had always used the malignant neoplasm section of ICD for topography, ACS based its topography on the malignant neoplasm section of ICD-8. A new edition of MOTNAC appeared in 1968, and was used extensively by cancer registrars.
In 1968, the International Agency for Research on Cancer (IARC) was asked by WHO to make recommendations about the content and structure of the neoplasm chapter for ICD-9 in consultation with the cancer and ICD units of WHO and various national bodies. Physicians expressed a desire for a cancer supplement that would also include morphology. Many consultants worldwide made suggestions for the neoplasm section of ICD-9 and emphasized the need for the coding of morphology or histology of tumors. The consultants suggested using the 1968 edition of MOTNAC as a basis for the morphology (histology) section: the morphology section of MOTNAC had been based on the neoplasm section of SNOP published by CAP. MOTNAC was widely accepted and translated into a number of languages.
Working parties for ICD-9 also recommended a requirement that the morphology of a tumor be recorded and coded. For many years, oncologists had realized that knowledge solely of the site or topography of a tumor was not sufficient for planning treatment or conducting research. For example, incidence and survival rates differ according to the histologic type of the tumor.
The working parties further recommended that a special adaptation of ICD, designated the International Classification of Diseases for Oncology , be created as the successor to MOTNAC for use by specialists in oncology who require greater detail of histologic classification. The recommendation was endorsed by a Study Group on the Classification of Diseases convened by WHO in 1971.
In 1976, WHO published the first edition of the International Classification of Diseases for Oncology, which had a topography section based on the malignant neoplasm rubrics of ICD-9 and a morphology section that expended the MOTNAC morphology code by one digit. CAP adopted the morphology of ICD-O for its revised edition of SNOP called the Systematized Nomenclature of Medicine (SNOMED) . The topography in SNOMED was again entirely different from that of ICD-O. Some of the SNOMED morphology terms for non-neoplastic tumor-like lesions and premalignant conditions are listed in ICD-O to help users differentiate these terms from those of true neoplasms. The SNOMED codes are no longer given because of continual changes to the codes, now principally published on the Internet. An ICD-O user simply needs to recognize that a term referenced in ICD-O-3 to SNOMED is not a neoplasm.
The Second Edition of ICD-O  was developed by a WHO/IARC working party and edited by Constance Percy, Valerie Van Holten, and Calum Muir. It was published by WHO in 1990 for use in cancer registries and by departments specializing in cancer begining with cancers diagnosed on January 1, 1992 through cases diagnosed on December 31, 2000. The Second Edition of the International Classification of Disease for Oncology is a dual classification and coding system for both topography and morphology. The topography code uses the same three- and four-character categories as ICD-10 for malignant neoplasms (C00-C80), allowing greater specificity for the site of non-malignant neoplasms than is possible in ICD-10. The Second Edition of ICD-O has been used extensively throughout the world and has been translated into many languages, including Chinese, Czech, French, German, Greek, Italian, Japanese, Portuguese, Russian, Slovak, and Spanish.
The Third Edition of ICD-O (ICD-O-3) has also been developed by a working party convened by WHO/IARC. The morphology codes for neoplasms have been revised, especially for lymphomas and leukemias. The codes incorporate the WHO classification [20, 21], which superseded the REAL (Revised European-American Lymphoma) classification for lymphomas  and the FAB (Frech-American-British) classification for leukemias . The Third Edition also recognizes the WHO classification of myeloid leukemias, including distinct combinations of morphology and cytogenetic abnormalities. An example is M-9863/3, chronic myelogenous leukemia, Philadelphia chromosome (Phl) positive, which is also referred to as chronic myelogenous leukemia, t(9;22)(q34;q11) or chronic myelogenous leukemia, BCR/ABL. ICD-O-3 was intended to be used in cancer registries throughout the world beginning with cancers diagnosed on January 1, 2001 and forward. However, there are a few countries that have decided to delay implementation of ICD-O-3 until 2003 and 2004 due to the many changes incorporated in ICD-O-3. Go to the ICD-O-3 training module.
Conversion algorithms (comparability codes) from ICD-O, Third Edition, to other coding systems are available. There is no change in topography between the Second and Third Editions of ICD-O, and the major changes in the morphology section are in the lymphomas and leukemias.