Colorectal cancer accounts for a substantial burden of morbidity and mortality in the population. While the need for reliable incidence data may be self-evident, the quality of cancer registration data has rarely been assessed. In Scotland during 1993, the medical records of a random sample of cancer registrations attributed to the year 1990 were sought. The sample contained 238 registrations of colorectal cancer, 217 (91%) of which had relevant medical records available for review. Registration details were reabstracted from available records and compared with data in the registry. Discrepancies in identifying items of data (surname, forename, sex and date of birth) were recorded in eight cases (3.7%, 95% confidence intervals 1.2-6.2%). None would have disturbed record linkage. Discrepancy rates of 3.7% (1.2-6.3%) in postcode of residence at the time of diagnosis (excluding differences arising through boundary changes), 8.3% (4.6-12.0%) in 'anniversary date' (excluding differences of 30 days or less), 2.8% (0.6-5.0%) in histological verification status and 13.5% (8.9-18.1%) in morphology code (excluding 'inferred' morphology codes) were recorded. Twelve cases (5.5%, 2.5-8.6%) were deemed not to warrant site codes for cancer of the colon (ICD-9 153) or rectum (ICD-9 154). In many respects, therefore, the data held about registrations of colorectal cancer in Scotland appear to show a high level of accuracy. Completeness of case ascertainment has still to be formally assessed.