Although application of topical psoralen followed immediately by ultraviolet-A irradiation (non-interval PUVA) was reported to be effective in the treatment of psoriasis, its precise mechanisms of action have not yet been explored. Since regular topical PUVA therapy, consisting of the topical application of psoralen followed by UVA exposure 1-2 h later, can change the number and morphology of Langerhans cells (LCs) and inhibit contact hypersensitivity (CHS), we investigated whether these same effects may be induced by non-interval PUVA. Our results showed that no differences exist between these two types of PUVA treatment. Non-interval PUVA treatments of 3 J/cm2 produced no erythematous reactions and resulted in changes in the number and morphology of LCs. The non-interval regimen also inhibited CHS to dinitrofluorobenzene applied to the treated skin by inducing the suppressor lymphocytes. These results suggest that there might be a link between the observed changes of the LCs and the effectiveness of non-interval PUVA therapy in the treatment of psoriasis, through a mechanism other than the inhibition of DNA synthesis of psoriatic keratinocytes.