The therapy of severe refractory psoriasis with the folic acid antagonist, methotrexate, recently received Food and Drug Administration approval. The folic acid antagonists were first used for psoriasis in 1951, and since then an extensive literature has attested the efficacy of methotrexate for treating various forms of psoriasis and psoriatic arthritis. Most studies indicate that about three fourths of patients treated for psoriasis enjoy good to excellent results. Methotrexate, it should be emphasized, is useful only for the control of psoriasis and is not curative. Psoriasis is a cutaneous disease characterized by a markedly increased rate of epidermal cell proliferation. Methotrexate, acting as an inhibitor of DNA synthesis by blocking dihydrofolic reductase, prevents cell reproduction in the psoriatic lesion, allowing the skin to return to normal function. Since methotrexate is administered systemically, other proliferating cell compartments, ie, bone marrow, gastrointestinal tract, and hair, are subject to possible drug effects. It