The change in colour of clinically unaffected psoriatic skin resolving concomitantly with distant sites of UV-irradiated psoriatic lesions was measured by a spectrophotometer. Skin colour was compared between psoriasis and an area of unaffected skin on a limb protected from UV light while the rest of the body received conventional treatment with UV light. Initially, there was a rapid loss of hue (represented by a flattening of the spectrophotometric curve) while unaffected skin showed elevated measurements which persisted during the course of treatment and subsided slowly for up to 2 weeks after cessation of UV light treatment. Clinically undetectable psoriatic lesions showed persistently raised spectrophotometric measurements for up to 10 weeks after cessation of treatment. This report suggests that a systemic factor stimulated or released by whole body UV light irradiation concomitantly influences the resolution of psoriatic lesions protected from the local effects of UV light.