Ataxia-telangiectasia is an autosomal recessive syndrome in which cancers develop in affected homozygotes at a rate approximately 100 times higher than in unaffected age-matched subjects. Retrospective studies have shown that persons heterozygous for the ataxia-telangiectasia gene, who make up about 1 percent of the general population, also have an excess risk of cancer, particularly breast cancer in women. Patients with ataxia-telangiectasia and cells derived from homozygotes and heterozygotes are unusually sensitive to ionizing radiation. Cancer incidence and mortality, mortality from ischemic heart disease, and mortality from all causes were compared prospectively for a mean of 6.4 years in 1599 adult blood relatives of patients with ataxia-telangiectasia and 821 of their spouses, who served as controls, in 161 families affected by ataxia-telangiectasia. In a case-control substudy, we compared documented occupational and fluoroscopic diagnostic exposures to radiation in the 19 female blood relatives in whom breast cancer was first diagnosed during the period of prospective observation with the exposures in 57 matched blood relatives who did not have breast cancer. Cancer rates were significantly higher in the group of blood relatives than in their spouses, specifically in the subgroup of 294 blood relatives who were known to be heterozygous for the ataxia-telangiectasia gene. The estimated risk of cancer of all types among heterozygotes as compared with noncarriers was 3.8 in men and 3.5 in women, and that for breast cancer in women was 5.1. Among the blood relatives, women with breast cancer were more likely to have been exposed to selected sources of ionizing radiation than controls without cancer (odds ratio = 5.8, P = 0.005). Male and female blood relatives also had 3-fold and 2.6-fold excess mortality from all causes, respectively, from the ages of 20 through 59 years. The ataxia-telangiectasia gene predisposes heterozygotes to cancer, particularly breast cancer in women. There is also excess mortality from all causes in adults under the age of 60. Diagnostic or occupational exposure to ionizing radiation probably increases the risk of breast cancer in women heterozygous for ataxia-telangiectasia.