Autoimmune processes are said to play an active role in aetiology of Behçet's disease (BD), which is also known as a multisystem disease. In the treatment of this autoimmune disease, cyclosporin A (CyA) is used. Gingival hyperplasia (GH) is one of the important side effects that have been observed in some of the patients. We aimed to evaluate the CyA-induced gingival hyperplasia in BD patients. There were 3 study groups, each having 5 patients. In the first group displaying GH, mast cells were located between epithelial cells and in the connective tissue. Mast cell granules were in crystalline form and electron-dense cored form. Fibroblasts and plasma cells were present in the connective tissue. The second group did not display GH and the mast cells were similar to those in the first group. The third group, was the control group, in which the mast cells were located between the epithelial cells and connective tissue. Mast cell granules were in electron-dense cored form. We concluded that the development of CyA-induced gingival hyperplasia is determined mainly by individual sensitivity to CyA, because although both test groups which received CyA showed an increased number and activity of fibroblasts, only one group of patients developed GH.