The purpose of this study was to assess the role of leukocyte adherence in the pathogenesis of the psoriatic lesion. Use was made of the fact that psoriasis and pemphigus differ considerably as to the presence of leukocytes in the respective lesions: abundance in psoriasis, and absence in pemphigus. The state of leukocyte adhesiveness/aggregation (LAA) was determined in the peripheral blood of 56 patients with psoriasis and 31 patients with pemphigus. Both classes of patients were subdivided into two categories according to the severity of the disease. It was found that in both diseases elevated values of LAA were obtained in the severe cases, whereas the mild cases did not differ significantly from normal controls. Thus, in psoriasis mean LAA values of 9.5% +/- 8% were recorded in the severe patients and 5.5% +/- 4.2% in the mild cases (p = 0.01), while in pemphigus the values were 15% +/- 9.6% and 6.6% +/- 3.7% respectively (p = 0.03). It is concluded that LAA per se does not play a primary role in causing the psoriatic lesion.