We searched for criteria that could indicate breast cancer families with a high prior probability of being caused by the breast/ovarian cancer susceptibility locus BRCA1 on chromosome 17. To this end, we performed a linkage study with 59 consecutively collected Dutch breast cancer families, including 16 with at least one case of ovarian cancer. We used an intake cut-off of at least three first-degree relatives with breast and/or ovarian cancer at any age. Significant evidence for linkage was found only among the 13 breast cancer families with a mean age at diagnosis of less than 45 years. An unexpectedly low proportion of the breast-ovarian cancer families were estimated to be linked to BRCA1, which could be due to a founder effect in the Dutch population. Given the expected logistical problems in clinical management now that BRCA1 has been identified, we propose an interim period in which only families with a strong positive family history for early onset breast and/or ovarian cancer will be offered BRCA1 mutation testing.