In an effort to determine if combination phototherapy (psoralens and UV-A [PUVA]) [PUVA-UV-B]) administered twice weekly provides a substantial advantage over therapy with PUVA alone, we investigated twice-weekly PUVA-UV-B therapy for the treatment of psoriasis. In a bilateral comparison study of 11 patients, we found that seven cases of psoriasis cleared in the same number of treatments with PUVA-UV-B or PUVA, but in four cases the PUVA-UV-B treated side cleared significantly more rapidly. In a study of these 11 split cases, plus ten additional patients treated with PUVA-UV-B twice weekly, this therapy was effective in clearing psoriasis. A bilateral comparison of "aggressive" and "nonaggressive" PUVA and UV-B increments in ten patients suggested that lower doses of PUVA and UV-B may, in the majority of patients, be equally effective. Further study is necessary to define the maximally effective protocol--maximizing clearance yet minimizing UV exposure, acute side effects, and long-term toxic reactions.