Background: Psoriasis vulgaris is a common chronic and inflammatory disease of skin that its etiology is not completely known. However, it seems that genetic and environmental factors play a role in this disease. In vitro studies show that prolactin as a neurohormone has an etiologic effect on psoriasis. This study aimed to compare serum prolactin levels in psoriatic patients with control group. Method: Prolactin level in serum was measured with ELISA method in 30 patients with psoriasis vulgaris and in 30 healthy persons as control group, which they had been matched with a view to age and sex. Result: Mean serum prolactin level in psoriatic patients was not significantly higher than control group. However, there was statistically significant relation between severity of disease and serum prolactin levels (r= 0.521, p=0.003). Conclusion: Positive relation between severity of psoriasis vulgaris and serum prolactin levels may suggest that prolactin is an index that shows severity of disease.