Measurement of the temperature change of skin exposed to ultraviolet radiation can be used to draw conclusions on the mechanism responsible for the development of ultraviolet erythema and on the question of which blood vessels are involved in the erythema. Under controlled environmental conditions we measured temperature change of skin areas, mainly on the back, irradiated with UVB (wavelengths between 280 nm and 315 nm) or UVC (wavelengths between 100 nm and 280 nm). Elevated skin temperature was measured after both types of irradiation. The influence of experimental conditions (ambient temperature, part of the body exposed to the radiation) was investigated. Ambient temperature did not influence the temperature difference measured significantly. The skin region used for the measurements did influence the results; temperature elevation after irradiation with UVC was markedly smaller on the forearm than on the back, for the same number of MED (minimal erythema dose; the dose required to elicit a just-perceptible redness of the skin). The fact that temperature elevations were found after UVC and after UVB irradiations points to involvement of arteriolar vessels in UVC as well as UVB erythema. The arteriolar dilation found speaks for the concept that both types of erythema are caused by a vasoactive substance, probably diffusing from the epidermis to the dermal blood vessels.