Mortality and cancer incidence were measured retrospectively in 263 ataxia-telangiectasia (A-T) homozygotes. For white and black A-T patients, respectively, all-cause mortality was 50 and 147 times higher than expected based on U.S. mortality rates. There were 52 primary cancers, representing a 61-fold cancer excess for white probands and a 184-fold excess for black probands. The cancer excess was most pronounced for lymphoma, with 252- and 750-fold excesses observed for whites and blacks, respectively. All the age-specific mortality and cancer incidence rates for blacks exceeded those for whites, and overall mortality was 3.0 times higher for black probands than for whites (P less than .001), whereas cancer incidence was 2.2 times higher (P less than .06). Among the white A-T patients, 36% of those who had died had lived at least until 20 years of age, and 33% of those still living were at least 20 years old.