Completeness of ascertainment of breast cancer cases by the Scottish Cancer Registry (SCR) was assessed by independent comparison with five clinical trials databases, comprising 2621 patients and spanning the total period of diagnosis 1978-2000. Overall, ascertainment was estimated to exceed 98%, although 0.3% of matched cases were misclassified as carcinoma in situ by the SCR. These results support the validity of national estimates of incidence of, and survival from, breast cancer in Scotland.