The International Classification of Diseases for Oncology (ICD-O) is a dual classification, with coding systems for both topography and morphology.

The topography code describes the anatomical site of origin of the neoplasm and, while it uses the same categories as in the neoplasm section of Chapter II of the International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10), some of the individual codes are different. The code always has a prefix of “C”, followed by a three-digit number that indicates the site (two digits) and the subsite (one digit), separated by a decimal point. For example, in C18.4, the C18 indicates that the site is the colon and the 4 indicates that the subsite is the transverse colon.

The morphology code describes the characteristics of the tumour itself, including its cell type and biological activity. The code is composed of four digits that indicate the cell type or histology and one digit that indicates the behaviour. The first four digits are separated from the last (behaviour) digit by a forward slash (/). The behaviour digit can be 0 (benign), 1 (uncertain behaviour), 2 (carcinoma in situ), 3 (malignant, primary site), 6 (malignant, metastatic site), or 9 (malignant, uncertain whether primary or metastatic site).

The search box, at the top of the page, allows the user to search for any term or partial term used in ICD-O-3 (any word or number, or part of a word or number) and can be used with or without a wild-card symbol (*). Thus, the following are all legitimate search terms: “colon”, “colo”, “colo*”, “*olo*”, “C18”, “adenocarcinoma”, “adeno”, “adeno*”, “*deno”, “8145”, “814”, “81*”, and “*14*”; such searches will return a list of all ICD-O-3 terms containing these words or numbers. Links from the returned terms will provide further definitions, including the synonyms and related terms.

Clicking on the Topographical codes tab at the top of the page will provide a list of all relevant ICD-O-3 codes in numerical order, from C00.0 to C80.9. (Note that the ICD-10 codes C43, C45, C46, C81-97, and D00-D48 are not part of the ICD-O-3 nomenclature for topography.)

The Morphological codes tab at the top of the page first provides an option to view all the codes listed in the original ICD-O-3 publication (2000), all the codes listed in the first revision of ICD-O-3 (2011), or all the codes added, changed, or revised in the first revision. Selecting one of these options will then provide a listing in numerical code order, from 8000/0 to 9992/3.

Any questions or comments about ICD-O-3 online should be sent to csu@iarc.fr or whofic@who.int