Epidemiologic data, family history, clinical data, HLA typing, neutrophilic chemotaxis, and immunofluorescence of clinically normal non-sun-exposed skin were studied in 46 Israeli non-Ashkenazi Jewish and Arab patients with Behçet's syndrome. HLA-B51 was present in 71 percent of the patient group as compared with 13 percent of the control group (relative risk = 17.1). In four of 30 families in the B51-positive group, there was a close relative of the proband with Behçet's syndrome who was carrying the HLA-B51 antigen. Neutrophilic chemotaxis in this group was enhanced in 80 percent of the patients, and in most patients no deposition of immunoglobulin in the dermo-epidermal junction was observed, whereas C3 was present in papillary vessels. In the B51-negative group, the family history was negative for Behçet's syndrome, neutrophilic chemotaxis was enhanced in only two of eight patients, and in four of six patients, IgM deposition was detected in the dermo-epidermal junction. It is concluded that in Israeli non-Ashkenazi Jews and Arabs, there is a significant association between HLA-B51 and the risk of developing Behçet's syndrome. The B51-positive patient group has a family history of the disease, enhanced neutrophilic chemotaxis, and a lack of immunoglobulin deposition in the dermo-epidermal junction.