Mast cells were counted in 122 patients with psoriasis and 80 healthy volunteers, at different times after scratch from 30 min up to 14 days. Koebner's phenomenon was induced in 74% of patients with active disease (63/85) and in 37.5% with stationary psoriasis (6/16). Mast cells were enumerated also in the skin of ten healthy controls, and their number on the inner aspect of the arm was found to range from 16 to 21 (mean, 17.6 +/- 1.65). Scarification increased the number of skin mast cells both in psoriatics and normal controls. The significant difference was noticed from the 4th day on after scratch, reaching a peak at days 14, simultaneously with the appearance of Koebner's phenomenon. In mature psoriatic lesions the number of mast cells was significantly lower than in newly formed psoriatic lesions, and it decreased progressively with effective anthralin treatment. It is concluded that mast cells play a role in skin response to trauma. In contrast to normal wound healing, their number is steadily increasing at the time of formation of the earliest psoriatic lesions.