Forty-nine patients with plaque-type psoriasis were enrolled in a randomized prospective trial that compared the efficacy of outpatient treatment with suberythemogenic doses of ultraviolet B (UVB) radiation administered with a tar oil to treatment with maximally erythemogenic UVB radiation with emollients. Seventeen percent of patients treated with tar oil and 26% of patients treated with maximally erythemogenic UVB failed to adhere to the protocol. Although the tar oil protocol seemed slightly less effective in those with more severe psoriasis, a roughly comparable fraction of patients who adhered to either protocol cleared at least 90% of psoriasis on surfaces exposed to UVB (76% vs 100%). The total dose of UVB to clearing was 44% less for patients treated with tar oil and suberythemogenic UVB. For most patients with moderate psoriasis, suberythemogenic UVB and tar oil is an effective, low-cost, and acceptable outpatient therapy.