A study to determine the accuracy of cancer mortality data was done using cancer deaths occurring during 1970 and 1971 in eight of the nine areas included in the Third National Cancer Survey (TNCS). Death certificates with an underlying cause of death of cancer were compared to the hospital diagnosis for 48,826 resident cases of single primary cancers. The underlying cause of death as coded on the death certificate was found to be accurate for about 65 per cent of the cancer deaths in this study. Misclassification problems occurred for colorectal cancer, the second leading cause of death from cancer. Colon cancer was overreported and rectal cancer was under-reported on death certificates. Other misclassification problems were found for cancers of the uterus, brain, and buccal cavity including most of its sub-sites. Physicians tended to report a non-specific site of cancer on the death certificate rather than the specific site identified by the hospital diagnosis.