Fifty xeroderma pigmentosum patients in Japan were examined for clinical characteristics and DNA repair of their cells, Skin cancers developed in 22 patients. Most of the patients without skin cancers were children, except for 5 older patients who had intermediate or nearly normal levels of DNA repair in their cells. All patients younger than 10 years old had no or very low activity of unscheduled DNA synthesis after ultraviolet light irradiation. Three genetic complementation groups, A, D, and E, and variants were found. Many Group A patients and no Group C patients characterized Japanese patients, compared with those in Europe and the United States, where Group C patients were most frequent. The high frequency of patients with low DNA repair capacities in their cells may account for the apparent high frequency of xeroderma pigmentosum patients in Japan. Age distribution of the cancer-bearing patients and their DNA repair characteristics suggest that almost all xeroderma pigmentosum patients will develop skin cancers unless their cells have nearly normal levels of DNA repair.