Although ultraviolet (UV) light is generally harmful to patients with systemic lupus erythematosus, most clinical and immunologic studies of UV exposure have evaluated the effects of UV-B (280-320 nm). The long-wavelength UV-A band (320-400 nm), however, is less toxic than UV-B and has different immunologic actions. Therefore, we studied the effect of UV-A irradiation on survival and immunologic function in the (New Zealand black x New Zealand white)F1 hybrid mouse model of systemic lupus erythematosus. Twenty-one (New Zealand black x New Zealand white)F1 mice were treated with 3.5 joules/cm2/day of UV-A light for 5 days each week, beginning at age 10 weeks. A control group consisted of 20 untreated animals. All UV-A-irradiated mice survived to 32 weeks, compared with 12 of 20 mice in the nonirradiated group (P = 0.0013). Splenomegaly was significantly decreased in the irradiated mice (P less than 0.03). Mice that received UV-A treatment combined with depilation had significantly improved lymphocyte responses to phytohemagglutinin and lipopolysaccharide and significantly decreased levels of anti-DNA antibodies compared with mice that received neither treatment. Reductions in spleen size and anti-DNA antibody titer were significantly correlated with improved parameters of lymphocyte function. These results suggest that a relatively small dose of UV-A exerts significant therapeutic action in murine lupus, perhaps through an effect on immunologic regulation.