At the Dead Sea spas, which are 400 m below sea level, both the ultraviolet radiation from the sun and the mineral-rich waters of the lake have special features. The separate influence of these two factors was studied in 81 patients with psoriasis. The condition was assessed during 4 weeks of treatment using the psoriasis area and severity index (PASI). In subjects who only bathed in Dead Sea water the improvement was 28%, in those who only sunbathed it was 73%, and in those who did both it was 83%. The main therapeutic factor was thus shown to be sun exposure, possibly enhanced by bathing in Dead Sea water. Similar degrees of improvement in spring, summer and autumn led to speculation about the optimum dose of solar radiation needed.