Mutations in the BRCA1/2 genes confer a high risk for breast and ovarian cancer, with usually adverse clinical characteristics. The clinical course and response to treatment in mutation carriers have been reported infrequently and are assumed to be worse than in sporadic breast cancer.Eleven members of an Eastern Finnish family with multiple cases of breast and ovarian cancers were screened for BRCA1/2 mutations using protein truncation test (PTT), conformation-sensitive gel electrophoresis (CSGE) and sequencing.Five of the six BRCA2 4088insA mutation carriers were affected. Mutation-positive breast/ovarian cancer patients had an excellent response to treatment even when prognosis as assessed by classical factors was poor.The 4088insA mutation appears to be associated with a favourable clinical course of breast and ovarian cancer, providing an informative example on the role of an individual mutation in assessing the prognosis for mutation carriers. Our results encourage more research on the effects of an individual mutation on clinical characteristics, response to treatment and outcome.