In order to assess the role of mast cells in the pathogenesis of mucocuta-neous lesions of Behçet’s disease, the following investigations were performed: (1) a comparative study of ultrastructural features of mast cells in sopntaneous and reactive (pa-thergic) lesions of the process and in apparently uninvolved skin of patients in the active stage of the disease; (2) comparative estimation of the histamine content at the site of cutaneous hyperreactivity lesions and in apparently uninvolved skin of the same patients. Various forms and degrees of degranulation were revealed among the mast cells. These forms were obviously more frequent in spontaneous and reactive lesions than in the uninvolved skin, 66 and 56% as compared with 7.7%. Also the histamine content was twofold more in the reactive lesions than in the unaffected skin of the same patient, 43 ng/mg tissue as compared with 21 ng/mg tissue. The results of our study seem suggestive of an active role of mast cells in this process. The role is probably similar to that of basophils in cutaneous basophilic hypersensitivity.