Background: The aetiopathogenesis of psoriasis is still not fully understood. Recently, it has been reported that prolactin (PRL) exerts a proliferative effect on human keratinocytes in vitro. PRL may, therefore, play an important role in the pathogenesis of psoriasis. Objective: To assess the serum PRL level in patients with psoriasis vulgaris (PV). Methods: Serum levels of PRL were estimated in 12 patients with PV (age: 11–45 years with mean ± SD 30.4 ± 10.2 years; sex: 7 males, 5 females) and the results were compared with those in 9 patients with atopic dermatitis (age: 15–47 years with mean ± SD 28.1 ± 11.9 years; sex: 4 males, 5 females) and 20 normal control subjects (age: 16–45 years with mean ± SD 36.1 ± 11.9 years; sex: 15 males, 5 females). Results: Serum PRL in PV (mean ± SD 25.8 ± 16.1 ng/ml) was significantly higher compared to those in atopic dermatitis (mean ± SD 9.1 ± 4.7 ng/ml) and normal control subjects (mean ± SD 10.3 ± 5.3 ng/ml; ANOVA → p = 0.0008). Three patients with PV (2 males and 1 female with ages of 35, 40 and 11 years, respectively) had the highest serum levels well above the normal range but they were <100 ng/ml, the minimum limit for the diagnosis of prolactinoma (χ² test → p <0.025). Conclusion: Since PRL belongs to the growth hormone family, its raised serum level may have a role in the hyperproliferation of kerationocytes in vivo, the hallmark of the psoriasis disease process.