The frequencies of HLA-class I antigens were studied in 39 patients with Adamantiades-Behçet's disease and were compared with those in 1415 [corrected] healthy controls; both patients and controls were of German origin. Moreover, the correlations of HLA antigens with onset, various clinical features and the course of the disease were investigated and the familial clustering of the disease was examined. We found an increased frequency of the HLA-B5 antigen in these series (P < 0.05, relative risk 2.6, confidence interval 1.2-5.5). The presence of HLA-B5 was more frequently detected in male (41%, relative risk 4.9, confidence interval 1.8-13.1) than in female patients (9%; P < 0.05). A significantly higher frequency of HLA-B5 was found in male patients with severe vascular involvement, including blindness and thrombosis, than in HLA-B5-positive male patients without these features. Incidences of genital ulcers, ocular involvement, cutaneous features, positive pathergy test and of secondary symptoms as well as the age of onset and the duration of development of the complete clinical picture were all found not to be significantly correlated with any of the HLA-A, -B, -C alleles. Familial clustering was also not found in German patients. Because of the low relative risk in the presence of HLA-B5 and the lacking familial clustering, it seems likely that there is a weak immunogenetically determined predisposition for Adamantiades-Behçet's disease in Germany. HLA-B5-positive male persons of German origin show a higher relative risk to develop or acquire Adamantiades-Behçet's disease than female ones.(ABSTRACT TRUNCATED AT 250 WORDS)