The International Agency for Research on Cancer (IARC) is sponsoring a worldwide study of childhood cancer incidence. Cancers in children are highly specific and differ in many ways from cancers found in adults. Data for the international study will be presented according to the classification scheme described below which has been devised specifically for use with paediatric cancers. The features of this scheme are: (1) it is based on the International Classification of Diseases for Oncology (ICD-O); (2) diagnostic groups are defined mainly in terms of morphology; (3) the common types of childhood cancer are individually specified; (4) certain other rare conditions of interest are distinguished; (5) it provides for flexibility of data presentation with respect to amount of detail; (6) all possible combinations of ICD-O morphology and topography codes are included; and (7) the maximum number of codes has been allocated to specific categories. There is a great need for standardization in the classification of childhood cancers and we propose that the scheme be used for presentation of incidence data for results of aetiological and other related studies of cancer in children.