The relationship of epidermal urocanic acid concentration and photoisomerization reactivity to human skin cancer was studied. Twelve cutaneous malignant melanoma patients, 10 basal cell carcinoma patients and 22 healthy matched controls were enrolled in the study. A solar simulating ultraviolet irradiator was used for phototesting the minimal erythema dose. Using the Finn Chamber technique, urocanic acid was sampled from the healthy skin of the upper back, prior to and after exposure to suberythemal UV doses. The mean values of total and trans-urocanic acid were higher in basal cell carcinoma patients than in controls, but this difference was not statistically significant. No corresponding phenomenon was evident in the case of cutaneous malignant melanoma patients and their controls. Photoisomerization induced by irradiation with 1 mJ/cm2 CIE (Commission Internationale de l'Eclairage) was statistically significantly lower in cutaneous malignant melanoma patients than in controls (p=0.04). A similar trend was seen in basal cell carcinoma patients vs. their controls, but the difference was not significant.