Hyperhomocysteinemia, a well-known and independent risk factor for cardiovascular disease, has been related in several studies with psoriasis patients. It has been suggested that homocysteine leads to endothelial dysfunction by causing an accumulation of asymmetrical dimethyl arginine (ADMA), a potent endogenous nitric oxide (NO) synthase inhibitor of the l-arginine–NO pathway. However, limited data is available regarding the psoriasis and ADMA relationship. In this study, we aimed to investigate the serum levels of homocysteine, ADMA and other metabolites from the l-arginine–NO pathway in psoriasis patients. Forty-two patients with chronic plaque psoriasis and 48 controls were enrolled in the study. Serum homocysteine, ADMA, l-monomethyl-l-arginine (l-NMMA), symmetric dimethylarginine (SDMA) and l-arginine levels, and l-arginine/ADMA ratios of psoriasis patients and the control group were measured. The severity of psoriasis was assessed by the psoriasis area and severity index (PASI). The mean ADMA and homocysteine values were significantly higher, and citrulline and l-arginine/ADMA values were significantly lower in psoriasis patients compared to control subjects. However, there were no significant differences among the patient and control groups with respect to mean SDMA, l-NMMA and l-arginine values. PASI scores strongly correlated with the ADMA level and moderately correlated with l-arginine/ADMA ratio. This study suggests that the l-arginine–NO pathway metabolites, especially ADMA, may play an important role in the pathogenesis of psoriasis. Additionally, serum ADMA levels of psoriasis patients may be an indicator of the disease severity.