To determine the efficacy of suberythemogenic ultraviolet phototherapy in conjunction with administration of a tar gel (SEUV TG), patients with widespread psoriasis were treated by application of a tar gel preparation followed after 12 hours by suberythemogenic doses of fluorescent sunlamp irradiation. In paired comparison studies, therapeutic effects of the following treatments were evaluated: SEUV-TG, a more conventional erythemogenic tar gel phototherapy regimen (modified Goeckerman), the tar gel alone, and SEUV irradiation following application of gel vehicle. Response to therapy was monitored with a severity score system. In patients with psoriasis responsive to phototherapy, smaller quantities of UV energy administered in combination with a tar gel were at least as effective as larger erythemogenic doses. Production of erythema with fluorescent sunlamp radiation does not appear to be necessary to improve psoriasis. Both UV radiation and tar gel have beneficial effects on psoriasis, but the combination is superior.