To assess the completeness and accuracy of notification of cancers by the National Health Service Central Register (NHSCR) for England and Wales.

Comparison of 720 cancer registrations ascertained from NHSCR up to May 1999 with those ascertained for the same cohort from six other sources and a pathology review of the NHSCR cancer registrations.

People born in Cumbria, north west England, 1950–89, and diagnosed with cancer throughout the UK, 1971–1989.

Cancer diagnoses notified by NHSCR differed substantially from those determined by this pathology review for 47 of the 688 notified cases reviewed (7%; 95% CI 5%, 9%). Over one third of these discrepancies were attributable to failures in data capture or coding by the cancer registration system and almost half to changes in diagnosis; 26 of the 47 discrepant cases were reclassified as non-malignant and 21 as malignancies but with a substantially different diagnosis. The 694 confirmed malignancies represented 94% (95%CI 92%, 95%) of the 740 cancers ascertained from all sources.

It is estimated that the cancer registration system missed at least 10% (95%CI 6%, 15%) of all incident cases of malignant disease. Without additional ascertainment from multiple sources and diagnostic review, it would be incautious to use NHSCR cancer registrations as the sole basis of an epidemiological study.