The role of the immune system in patients with psoriasis vulgaris (PV) was investigated. The genetic and immunological basis for psoriasis is still unknown. Because of the reports of immunological defects in this disease, we investigated serum levels of immunoglobulins IgG, IgM, IgA, complement proteins C3, C4, serum zinc (Zn) levels and natural killer (NK) cell activities. Skin lesions of the psoriatic patients involved in the study comprised less than 10 % of the total body and the disease was in a stationary period. Zn levels were measured by atomic absorption spectrophotometry. NK cell activity was measured by Cr-51 (Na-2 (CrO4)-Cr-51). IgG., IgA IgM, C3 and C4 assays were done by liquid-phase immunoprecipitation assay with nephelometric endpoint detection. IgG, IgA, C3 and C4 levels were significanty higher in patients with PV than in healty controls (p<0.05). However, NK cell activity, serum Zn and IgM levels did not show significant differences between these two groups. There are changed immunological responses, which may play an important role in the pathogenesis of the disease. Many controversial results have been related to immunological parameters in psoriatic patients. Therefore, more detailed studies in this field need to be done to determine the relationship between psoriasis and the immune system.