Abstract— Urocanic acid (UCA) is a major chromophore for UV in the skin and has been suggested to act as an initiator of UV-induced immunosuppression. It converts from the naturally occurring trans-isomer to the cis-isomer on UV exposure. Isomerization is dose dependent until the photostationary state is reached, and the seasonal variation in irradiance from the sun may lead to changes in the percentage of UCA present as cis -UCA throughout the year. Thirty young healthy subjects, skin types I-IV, were followed from early summer till spring. At each of six visits (June, July, August, October, December, March), pigmentation and the concentration of UCA isomers were measured at six body sites: forehead, upper chest, upper back, outer upper arm, inner upper arm and buttock. In exposed as well as unexposed regions a variation in pigmentation was found, peak values being recorded in August. Total UCA concentration was lower in July and August than in the rest of the year, irrespective of body site. In July, the percentage of cis -UCA was close to the maximal obtainable (50-60%) at all sites except the buttock. In the three winter months the percentage of cis-UCA was below 7% in all regions except for the forehead, where the mean cis -UCA was 18% in October and March. No consistent relationship was found between UCA isomers and pigmentation or skin type.