The familial occurrence of Behçet's disease in the 564 Behçet's patients treated in the Uveitis Survey Clinic of the Hokkaido University Hospital between 1967 and 1992 was investigated. Familial occurrence of Behçet's disease was found in 18 patients from 9 families. The male-to-female ratio was 3.5 in the familial cases, which was significantly higher than in the sporadic cases. Familial occurrence was most frequently seen among siblings. Most patients, including all female patients, had the complete type. They had typical ocular lesions, and exhibited bilateral uveoretinitis with poor visual prognosis, particularly the female patients. Frequency of HLA-B5 was 92% in the familial patients, which was much higher than in both the controls and the sporadic cases. It seems probable that patients with familial occurrence of this disease have a poorer prognosis with more susceptible immunogenetic backgrounds.