Recent evidence suggests that the angiotensin converting enzyme (ACE) is present in skin. The real value of the determination of ACE activity as a clinical-biochemistry test for the diagnosis of psoriasis has not been attained. Serum and tissue ACE were measured in 60 patients with psoriasis, 20 patients with lichen planus, 20 patients with seborrhoic dermatitis and in 20 healthy individuals. The serum and tissue ACE activity was determined before and after therapy, using the spectrophotometric method and hippuryl-l-histidyl-l-leucine as a substrate. The results showed that serum ACE activity before therapy was significantly increased in both groups--patients with psoriasis (p < 0.001) and patients with lichen planus (p < 0.001) in comparison to healthy individuals. However, there were no significant differences in serum ACE activity among patients with seborrhoic dermatitis and healthy individuals. After therapy, serum ACE activity significantly decreased in both groups of patients with psoriasis and patients with lichen planus comparing it to the level found in the control group. The values in both were similar. The tissue ACE activity in altered skin was significantly increased only in the patients with psoriasis in comparison to uninvolved skin of these patients, as well as the skin of healthy individuals. After therapy, there were no significant differences in tissue ACE activity between the treated skin and the healthy skin. In conclusion, determination of tissue angiotensin converting enzyme activity can be used in the differential diagnostic of indistinct clinical forms of psoriasis.