Although application of tar products and subsequent exposure to ultraviolet radiation (the Goeckerman regimen) has repeatedly been demonstrated to be effective therapy for psoriasis, the therapeutic role of each component has remained uncertain. Utilizing the bilateral comparison technique in 30 hospitalized patients with chronic stable plaque-type psoriasis vulgaris, we closely monitored the clinical responses to ultraviolet radiation (Westinghouse fluorescent FS40 bulbs, 290--400 nm) and a variety of tar preparations and lubricant vehicles in combination and separately. We found that: 1) 4 weeks of maximally-aggressive exposure to ultraviolet radiation alone will markedly improve, but not completely clear, psoriasis unless combined with a tar preparation or lubricating base; 2) 5% crude coal tar plus ultraviolet radiation offers no clear advantage or benefit over lubricating base plus ultraviolet radiation; and 3) none of the tar preparations tested offered any consistent advantage over any other preparation.