Cancer registries are essential in order to monitor the incidence of cancer and, with proper follow-up, survival in a population. However, the usefulness of the registry depends upon the data quality. To validate the Danish Cancer Registry concerning breast cancer in female residents of Aarhus county 1983-1989, registry records were compared with clinical records. Completeness was validated in 2062 patients and correctness was determined for pathologically proven primary invasive breast cancer in 1949 patients. Incidence data were complete with no tumours missing. Data were coded according to ICD-7 and correctness of registry data in terms of basis of diagnosis and tumour malignancy was 99%. Information on extent of disease (stage) was not complete and there was a high disconcordance, in particular for bilateral breast tumours and cases with distant metastasis. Even crude staging into local and regional disease was inconsistent for 13% of cases, which probably hampered survival analysis by stage. The quality of registry data should be addressed when using variables, such as stage, not routinely reported by registries.