In South Wales cancer registration is done principally by means of the Hospital Activity Analysis. Altogether 1460 hospital records of cancer patients (19% of the 1972 registrations received by May 1973) were studied and the principal items of information required for cancer registrations by the Office of Population Censuses and Surveys were copied and subsequently compared with the corresponding registrations at the Welsh Hospital Board's cancer bureau. Differences between these 're-registrations' and the original registrations were analysed item by item. There were 234 registrations with errors in the diagnostic summary (although 110 of these would cause misclassification only under the fourth digit of the ICD code), 164 with errors in date of birth (36 of which would cause classification in the wrong WHO age group) and 198 with errors in the date of registration (112 of which were wrongly ascribed to the year 1972). Error and omission rates were particularly high for NHS number, occupation, place of birth, and histology.