Using repeated minimal erythema dose (MED) testing, we investigated the magnitude and duration of tolerance to short-wave UV radiation in the B range (UV-B) in 37 patients with psoriasis who received at least 12 UV-B phototherapy treatments. Without substantial erythema developing, half of the patients received UV-B doses in excess of 13 times their pretreatment MED dose and a fourth received UV-B doses in excess of 28 times their pretreatment MED dose. On average, tolerance faded to about half that present at the last UV-B treatment in 3 weeks. Six weeks after therapy was stopped, most posttreatment MEDs were less than twice the pretreatment MED. The magnitude of tolerance achieved and the rate of decay did not vary with skin type. Our findings indicate that repeated exposures to suberythemal or mildly erythematous doses of UV-B can induce exceptional degrees of tolerance to UV-B radiation. The duration of this increase in tolerance is, however, short.