Thirty-eight patients with plaque-type psoriasis were enrolled in a double-blind psoralen plus ultraviolet A (PUVA) treatment study comparing the efficacy and side effects of 5-methoxypsoralen (5-MOP) and 8-methoxypsoralen (8-MOP). Patients treated with 8-MOP healed significantly faster than those on 5-MOP for 6 weeks of treatment, but there was no significant difference after 9 weeks. There was no significant difference in side effects between the two groups, but nausea tended to be more common in the 8-MOP group. One patient on 5-MOP had signs of toxic hepatitis. The importance for maximizing absorption of taking 5-MOP with food is stressed, and PUVA treatment should be given 3 h after intake of the drug.