We investigated DNA damage by the number of micronucleated binucleated cells (MNBC) in lymphocytes, serum homocysteine, vitamin B12, folic acid, and two polymorphisms (C677T and A1298C) in the MTHFR gene in 35 patients with exacerbated psoriasis vulgaris classified according to the psoriasis area and severity index (PASI) score and treated by GT. The gene discussed is MTHFR; the disease is psoriasis vulgaris.