Ethanol has been reported to exacerbate psoriasis. Since immunological mechanisms are considered to be important for the pathogenesis of psoriasis, we compared the effects of ethanol on lymphocyte proliferation in 15 healthy control individuals and 15 patients with psoriasis. We employed the spontaneous and phytohemagglutin in (PHA)-induced uptake of 3H-TdR to measure lymphocyte proliferation. Ethanol was added to cultures at concentrations ranging from 0.5 to 0.0005% (vol./vol.). We found that both spontaneous and PHA-driven lymphocyte proliferations were significantly lower in patients with psoriasis (P < 0.002). Spontaneous blastogenesis in both controls and patients remained stable under ethanol. In controls, ethanol suppressed the PHA-driven lymphocyte proliferation in a dose-dependent fashion. By contrast, in patients with psoriasis ethanol significantly increased lymphocyte proliferation by 2-3 times (p < 0.002). Our data indicate that in psoriasis the lower lymphocyte transformation is abnormally enhanced by minimal doses of ethanol.