PURPOSE: To determine the epidemiology of Behcet's disease in Western Switzerland and to study the characteristics of ocular disease. METHODS: Patients newly diagnosed with Behcet's disease, seen from January 1990 to December 1998 at Hopital Jules Gonin, La Source Eye Center, and the Division of Immunology and Allergy, University of Lausanne, were analyzed. The Japanese diagnostic criteria for Behcet's disease were used and both complete and incomplete forms were included. Incidence, proportion of cases with ocular involvement, and the characteristics of ocular disease were determined. RESULTS: During the follow-up period, a total of 35 Behcet's patients were seen in the three study centers. Twenty-five of these patients had ocular involvement. The incidences of Behcet's disease and Behcet's ocular disease in the western part of Switzerland, with a referral population of roughly 600000 persons, were 0.65 and 0.46 per 100000 per year, respectively. The mean age of the group was 33.2 years, age at presentation was 25.8 years, and mean follow-up was 5.7 years. The most frequent sign was oral aphthous involvement (86%), followed by ocular manifestations (72%). The HLA-B51 antigen was present in 66% of the patients with ocular involvement. Panuveitis was present in 20/25 patients (80%), anterior uveitis in 3/25 patients, and posterior involvement in 2/25 patients. Twenty-two patients (88%) had systemic treatment that always included corticosteroids; it was associated with other treatments in 17/22 patients. Mean duration of treatment was 45.5 +/- 25.1 (SD) months. The mean recurrence-free period in the six patients in whom treatment could be interrupted was 18.0 +/- 8.8 (SD) months. Visual function was maintained at the end of the follow-up for the collective and visual outcome was correlated to visual function at presentation. CONCLUSIONS: The incidence of Behcet's disease in Western Switzerland was comparable to that in other western nonendemic countries. With early diagnosis and appropriate therapy, the prognosis was favorable in our collective of patients.