Fibroblast cultures of seven patients with xeroderma pigmentosum (XP), 19 healthy sibs or parents of XP patients (XP-heterozygotes), and 24 healthy normal controls were studied for chromosome instability induced by ultraviolet rays (UV). We used a UV source that contained predominantly UV-A and UV-B at an intensity of 500 J/m2 and evaluated the induction of micronuclei (MN) and sister chromatid exchange (SCE). the XP homozygotes had a UV sensitivity that was clearly above that of all heterozygotes and normal controls. Heterozygotes had an increased rate of UV-induced MN (4.76 +/- 1.96 vs. 1.82 +/- 2.05, p less than 0.0001) and increased UV induction of SCE (13.21 +/- 3.49 vs. 9.01 +/- 1.25, p less than 0.001), as compared to normal controls. These data support epidemiologic findings that suggest that XP heterozygotes are particularly cancer prone. In addition, the determination of the UV sensitivity in vitro as described may be used for genetic counseling of asymptomatic relatives of XP patients.