Psoriasis vulgaris (PV) is a chronic skin disease with unclear pathogenesis. In the present study we investigated the effect of systemic photochemotherapy (PUVA therapy- psoralen and UVA therapy) on the expression of IFN-γ, IL-12p40 and IL-23p19 in lesional psoriatic skin. Fifteen patients with chronic plaque type psoriasis selected to be treated with PUVA therapy were recruited for this study. Expression of IFN-γ, IL-12p40 and IL-23p19 in psoriatic lesions before and after twenty PUVA treatments was established by using immunohistochemistry (IHC). A significant decrease in expression (p < 0.05) of IFN-γ, IL-12p40 and IL-23p19 in epidermis and dermis of psoriatic lesions was observed. The immunosuppressive effect of PUVA therapy presented with decreased expression of biologically active IL-23 (IL-12/IL-23p40 + IL-23p19) as a part of the Th17 pathway, and IFN-γ (Th1 pathway) led, in our patients, to a marked clinical improvement as shown by PASI (before therapy 20.55 and after therapy 3.33).