The cytogenetic response to G2-phase X-irradiation was examined in phytohemagglutinin-stimulated peripheral-blood lymphocytes from 69 individuals, a few of whom were cancer patients. The cancer patients had not received radiation or chemotherapy. The responses of cells arrested by Colcemid 30 to 90 min after X-irradiation (58R) could be divided into 2 distinct categories: 51 individuals had aberration frequencies typical of normal individuals in previous studies, while 18 others had a 2- to 3-fold higher frequency of chromatid breaks and gaps. Because chromatid breaks and gaps result from unre paired DNA strand breaks, the first category may represent an efficient DNA repair phenotype, while the second may repre sent a deficient repair phenotype. The individuals with the deficient G2 response reported having first- and second-degree relatives with a 3.6- and 2.2-fold higher mean frequency of cancer, respectively. The present results, together with those from earlier studies of families with a genetic disorder predispos ing to cancer, suggest that this deficient cytogenetic response to G2 phase X-irradiation is associated with a high risk of cancer.