Patients who are homozygous for ataxia-telangiectasia (AT) have an exceptionally high incidence of cancer. Heterozygous individuals for the disease have been reported to be at an increased risk of cancer, particularly breast cancer in female carriers. We have analyzed eight Norwegian families with AT for cancer incidence in the parents, in the parents' sibs, grandparents, and grandparents' sibs. Two of the obligate heterozygote females have had premenopausal breast cancer. This incidence is significantly higher than expected for that group. No increase in the cancer incidence was observed in the parents' sibs, the grandparents, or the grandparents' sibs. Since the incidence of AT is low, data from many sources have to be combined to allow any conclusion.