The multifocal involvement in Behçet's syndrome was grouped into a spectrum of four types, three of which appeared to have an immunogenetic basis. HLA-B5 was related to the ocular type of Behçet's syndrome (relative risk 7.3), HLA-B27 to the arthritic type (relative risk 12.1) and HLA-B12 to the muco-cutaneous type (relative risk 3.9). The concept that recurrent oral ulceration and Behçet's syndrome may belong to a disease spectrum is substantiated by the natural course of the disease. Furthermore, patients with recurrent oral ulcers share with the muco-cutaneous type of Behçet's syndrome a significantly increased frequency of HLA-B12 (relative risk 2.6). The HLA markers may also prove to be significant in the differential diagnosis and prognosis of a disease which may present under a confusing variety of clinical manifestations.