Excision repair of pyrimidine dimers induced by simulated solar radiation in the skin of patients with basal cell carcinoma. One prominent lesion induced in DNA by ultraviolet (UV) radiation is the cyclobutyl pyrimidine dimer formed between adjacent pyrimidines on the same DNA strand. We investigated whether people who have developed basal cell carcinoma on sun-exposed skin have an altered ability to repair UV-induced pyrimidine dimers in DNA. Twenty-two patients with at least one basal cell carcinoma, aged 31-84 years, and 19 healthy volunteers, aged 25-61 years, took part in the study. Both groups were given one minimal erythema dose (MED) of simulated solar radiation on the lower back. DNA was extracted from the irradiated skin 0 to 6 h later, and the number of UV-induced pyrimidine dimers was determined using a dimer-specific endonuclease. At time 0, the average number of dimers per unit of DNA was similar in the two groups. After 6 h, an average of 22 +/- 4% of the dimers were removed in the group with basal cell carcinoma compared to 33 +/- 4% in the cancer-free group. In the basal cell carcinoma group, only 23% of the patients repaired more than 30% of the dimers after 6 h, compared with 53% of the cancer-free subjects (p less than 0.05). We conclude that patients who develop basal cell carcinoma on sun-exposed skin may have a decreased ability to repair pyrimidine dimers induced in skin exposed to simulated solar radiation. 