Freckles were induced experimentally by single exposures of lower back skin to ultraviolet radiation. The minimal freckling dose was between 6 and 10 times the minimal erythema dose. Repeated less intense exposures did not induce freckles. Freckles could be induced only in subjects with natural freckles on exposed skin. Freckles induced by single exposures to ultraviolet radiation have distinctive characteristics which enable their clinical recognition. We have called these "sunburn freckles".