Mycophenolic acid (MPA), an inhibitor of purine synthesis, was evaluated for its therapeutic and adverse effects in 29 patients with psoriasis. MPA was administered orally for at least 12 weeks, during which time the daily dose was increased from 1600 to 4800 mg depending on occurrence of adverse reactions. Complete clearing occurred in 1 of the patients, almost complete clearing in 14, definite improvement in 13, slight or doubtful improvement in 1. The full effect of MPA required a median time of 8 weeks (range 5-14). After discontinuing MPA, relapses began at a median time of 4 weeks (range 3-8). The severity of psoriasis was scored on a 0 to 108 scale using a newly devised system. The mean severity and range before treatment was 47 (21-88); after 12 weeks, 15 (0-50). Adjustment of dose on the basis of side effects resulted in a median daily dose of 3600 mg (range 2400-4800 mg; 30-96 mg/kg ideal weight). Characteristic dose-limiting side effects were soft or frequent bowel movements, diarrhea, nausea, and anorexia. One instance of reversible, dose-related leukopenia was identified.