About 1.4% of the general population are heterozygous carriers of the gene for ataxiatelangiectasia (A-T), an autosomal recessive progressive neurologic syndrome in which cancer incidence of homozygotes is approximately 100-fold greater than the general population's rates. The hypothesis that A-T heterozygotes are predisposed to breast cancer was tested by the unbiased statistically powerful index-test method based on molecular genotyping. The A-T gene carrier status of 775 blood relatives in 99 A-T families was determined by tracing the A-T gene in each family through tightly linked flanking DNA markers. There were 33 women with breast cancer who could be genotyped; 25 of these were A-T heterozygotes, compared to an expected 14.9 (odds ratio 3.8, 95% confidence limits 1.7-8.4, one-sided p = .0001). This demonstrates that the A-T gene predisposes heterozygotes to breast cancer. For the 21 breast cancers with onset before age 60, the odds ratio was 2.9 (1.1-7.6, p = .009) and for the 12 cases with onset at age 60 or older, the odds ratio was 6.4 (1.4-28.8, p = .002). Thus the breast cancer risk for A-T heterozygous women is not limited to young women but appears even higher at older ages. Of all breast cancers in the United States, 6.6% may occur in women who are A-T heterozygotes. This proportion is several fold greater than the estimated proportion of carriers of BRCA1 mutations in breast cancer cases with onset at any age.