Methods We studied 107 Ashkenazi Jewish women with breast cancer seen at medical oncology acid genetic counselling clinics in New York over a three and a half year period beginning in 1992. 80 of the women were diagnosed before age 42 years; the other 27 were diagnosed between 42 and 50 years and had a positive family history, Genomic DNA testing by PCR amplification was done to identify any 185delAG mutations of the BRCA1 gene.

Findings Of the 80 women diagnosed before the age of 42 years, 16 (20%, 95% CI 11 . 2-28 . 8) were heterozygous for the mutation, All 16 women had at least one first-degree or second-degree relative with breast or ovarian cancer, Of 27 probands diagnosed with breast cancer between the ages of 42 and 50 years who had at least one first-degree relative affected with breast or ovarian cancer, 8 (30%, 95% CI 12-47) had 185delAG mutations,

Interpretation These data suggest that screening for the 185delAG mutation may be useful in genetic counselling of these women where options for detection and prevention of possible cancers can be discussed.