Psoralens are metabolized in the liver and thus can produce liver damage in laboratory animals when given in excessive doses. However, in humans, reports of the effects of psoralens on liver functions during photochemotherapy have been contradictory. We studied 311 patients t o observe the effects of various phototoxic drugs: 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP), 4,5,8-trimethylpsoralen (TMP)-on liver function during photochemotherapy. Of the 311 patients, only seven had transient elevations of transaminases. Incidence of hepatotoxicity of 8-MOP, TMP and 5-MOP showed 0.6%, 6% and 3%, respectively. Although the effects of psoralens on liver functions during photochemotherapy have been contradictory in humans and there have been few cases showing transient elevated values of liver transaminases during photochemotherapy, it is advisable to obtain serial liver function tests before and during photochemotherapy.