In order to find biomarkers to measure the effects of UV irradiation, we examined the accumulation of p53 protein and pyrimidine dimers in 18 solar keratosis specimens. Frozen or AMeX-fixed solar keratosis specimens were immunohistochemically stained by anti-p53 mouse monoclonal antibody, pAb1801 and polyclonal anti-(pyrimidine dimer) antibody. Nuclear accumulation of p53 protein was found in 5/18 (28%) solar keratosis lesions. The percentage of cases showing nuclear p53 protein varied according to the histological type; in the bowenoid type it was 4/7 (57%); in the atrophic type it was 1/7 (14%). Nuclear pyrimidine dimers were not stained in solar keratosis, although the skin of UV-irradiated nude mice was positive. Accumulation of p53 protein is a good marker for early precancerous change caused by UV exposure.