Women at risk for inherited breast cancer have been evaluated in two collaborating Norwegian cancer genetics centres and offered follow-up in the out-patient clinics of all major Norwegian hospitals for the last 11 years. The families were identified on the basis of clinical criteria. The breast cancer genes BRCA1 and BRCA2 were identified in 1994-95. Even though several hundred different mutations in these genes have been described, a significant proportion of Norwegian families with breast cancer appears to have a few frequent mutations. This most probably is a result of the changes in the population structure of Norway as the population went through a genetic bottleneck during the Black Death, which was followed by rapid population expansion. Mutation analysis has now been put in use to identify Norwegian breast cancer families. Such analysis should be offered to all Norwegian patients with breast or ovarian cancers regardless of age of onset or positive family history. For the time being, analysis should be restricted to the detection of the demonstrated frequent mutations in BRCA1.