Eight breast cancer pedigrees with a high probability of containing individuals with the BRCA1 gene mutation (odds 79·2–99·9 per cent) were identified through genetic linkage analysis using probes located within q12–22 on the long arm of chromosome 17. Some 102 female relatives were successfully typed with one or both of adjacent markers D17S588 and D17S579, and 41 were probable non-BRCA1 mutation carriers. Of the remaining 61 women classified as probable BRCA1 carriers, breast cancer was diagnosed in 35. As expected from epidemiological segregation analysis studies, 13 of these had bilateral disease. Approximately two-thirds of women unaffected by malignancy and alive at the time of observation were non-BRCA1 carriers. Lifetime disease penetrance of the BRCA1 gene was 88 per cent and this plateau was reached earlier (by age 65 years) than that estimated in segregation analysis. The survival curve of patients with breast cancer was less steep in BRCA1 gene carriers than that in the general population; 5-, 10- and 20-year survival rates unadjusted for non-cancer deaths were 83, 63 and 41 per cent respectively. The 5-year survival rate was significantly higher in BRCA1 carriers than that in an age-matched Scottish population (P<0·05).