The mutation frequency of BRCA1 and BRCA2 in women with breast cancer varies according to family history, age at diagnosis and ethnicity. The contribution of BRCA1 and BRCA2 mutations in breast cancer populations, unselected for age and family history, has been examined in several studies reporting mutation frequencies between 1% and 12% by screening methods, population sizes, and to what extent the gene/s were screened differed in the studies. We wanted to clarify the proportion of breast cancer attributable to mutations in BRCA1 in an unselected breast cancer population from the Stockholm region. All incident breast cancer patients treated surgically in a 19-month period were eligible for the study and 70% (489/696) participated. Exon 11 of BRCA1 was screened for mutations using the protein truncation test, and the mutation frequency was estimated from that. In previous studies on high-risk families from Stockholm, more than 70% of the mutations were detected in exon 11. Two mutations were found, both in patients with a family history or their own medical history of ovarian cancer, giving a mutation frequency in exon 11 of 0.4% and an estimated BRCA1 mutation frequency of <1%. Mutations in BRCA1 in unselected breast cancer cases in our region are rare and likely to be found only in high-risk families. Our BRCA1 prevalence is the lowest of all studies on unselected breast cancer patients, probably reflecting the comparatively low rates detected also in high-risk breast cancer families from the region.