Urocanic acid (UCA), present in the stratum corneum as trans-UCA, absorbs ultraviolet (UV) radiation and isomerizes to cis-UCA. Cis-UCA has been demonstrated to initiate suppression of selected immune responses in several experimental systems. Topical application of UCA-containing products reduces UV-induced erythema, but a role for endogenous UCA in photoprotection has not been reported. In this study the relationship between UCA isomers, pigmentation, minimal erythema dose (MED), and stratum corneum thickness was investigated. Pigmentation, concentration of total UCA, and the percentage present as the cis-isomer was measured in 36 healthy subjects, skin type I-IV, at six UV-exposed and nonexposed body sites: forehead, chest, back, outer upper arm, inner upper arm, and buttock. The MED was determined by phototesting on buttock skin, and a punch biopsy for measurement of stratum corneum thickness was taken adjacent to the site of the phototest. The percentage of cis-UCA was significantly higher in UV-exposed than on nonexposed areas. A small intraindividual variation in total UCA was found, being high on the buttock and the arm, lowest on the forehead. The subject to subject variation of total UCA was considerable at all body sites. No correlation was found between total UCA and MED, skin type, pigmentation, or stratum corneum thickness.