The action spectrum for the induction of phototoxic erythema by treatment with 8-methoxypsoralen (8-MOP) and UVA peaks at around 335 nm. Because earlier investigations reported peak phototoxic activity at 365 nm, we compared the antipsoriatic efficacy of 335 and 365 nm in an oral psoralen photochemotherapy (PUVA) regimen using a monochromator. PUVA with 335 nm was twice as effective as with 365 nm, with respect to both erythemogenicity and the cumulative dose required for clearing psoriasis. However, 335 and 365 nm were equally effective if delivered in equal erythema doses. It appears that in human skin the antipsoriatic activity of 8-MOP parallels its erythemogenicity.