We evaluated the accuracy and time to reporting of cancer diagnoses obtained through the Finnish Cancer Registry (FCR) for the Alpha-Tocopherol Beta-Carotene Cancer Prevention (ATBC) Study in 1985-1997. In the ATBC Study suspect neoplasms were centrally reviewed through medical records and pathology specimens. The FCR data were compared against the reviewed data for 3600 cancers of eight sites. For most sites, 95% of the cases were reported to the FCR within 0.9 years with longer delays for lung and pancreatic cancers. Ninety-six percent of all FCR cases received the same primary site diagnosis in the ATBC review, and in 1.4% no malignancy was found. Conversely, 97% of cancers ascertained in the ATBC review had the same primary site in the FCR and 0.8% were unknown to the Registry. The accuracy of the FCR data is high but the delay in case notification should be considered in epidemiological studies.