Assessment of risk of developing hereditary cancer and subsequent clinical surveillance is largely based on family history. It is considered standard practice to confirm as many diagnoses as possible in cancer families. Our aim was (1) to assess inaccuracies in reporting of cancers by families, (2) to assess the need for confirmation of diagnosis, and (3) to estimate how many families would have been entered unnecessarily or excluded from screening. A retrospective study of 595 case notes was performed in two centres. Methods of confirmation included information from the cancer registries, death certificates, hospital notes, and histopathological records. Accuracy varied by site of cancer and by the closeness of the relationship to the affected person. Reported abdominal malignancies were inaccurate in 20%, whereas 5% of reported breast cancers were inaccurate. In two families the family history of cancer proved fictitious. Management was altered in 231213 (11%) families following cancer confirmation. The results of the study favour verification of cancer diagnoses particularly if decisions regarding surveillance or prophylactic surgery are based on the family history.