Objective. Behcet's disease (BD) is a systemic inflammatory disorder polarised to the Th1 and Th17 immune systems. Allergic diseases are polarised to the Th2 immune system. The aim of the present study is to investigate the prevalence of allergic diseases in patients who have BD. Methods. The study involved a large-scale interview survey of Japanese patients with BD at 21 institutes of ophthalmology; 353 patients (255 males and 98 females) were recruited for this study. We analysed the history of allergic diseases such as atopic dermatitis (AD), allergic rhinitis (AR), bronchial asthma (BA) and drug/food allergies (FA). Results. Oral aphthous ulcers, ocular lesions, skin lesions, genital ulcers, arthritis, neurological lesions, intestinal lesions, deep vein thrombosis and epididymitis were reported in 95.8%, 98.6%, 72.5%, 44.8%, 13.9%, 6.8%, 6.2%, 3.7% and 1.4% of the patients, respectively. It was also reported that 73 patients (20.7%) had histories of allergic diseases: AD (5 cases, 1.4%), AR (36 cases, 10.2%), BA (19 cases, 5.4%) and FA (30 cases, 8.5%). This percentage was significantly lower than in a survey that Japan's Ministry of Health, Labour and Welfare conducted for healthy population (47.6%) (odds ratio = 0.29, 95% confidence interval = 0.22-0.38, p=4.9 x 10(-22)). Frequencies of posterior/panuveitis, relatively severe ocular findings, and visual prognosis were not affected by a history of allergic diseases in BD. Conclusion. Patients with BD had fewer complications from allergic diseases than did the entire population of Japan.