The mechanism of skin hyperreactivity (pathergy) in Behçet's disease is unknown. It has been suggested that the response is due to an Arthus-like reaction. In 18 patients with active Behcet's disease, pathergy was induced and the skin biopsied after 24 h. The histological reaction was that of mold to intense round cell infiltration. perivascular in type. Direct immunofluorescence failed in all 18 patients to demonstrate the local presence of immunoglobulins and complement at the site of lesion. A marked increase in the number of mast cells was observed at the site of reaction and scattered throughout the dermis, the role of which is to be further investigated. The study failed to demonstrate a humoral mechanism in the production of the cutaneous hyperactivity.