Abstract: This is an analysis of adult Behcet’s disease (BD) in Iran, from the Iran Registry of 7187 BD patients, gathered from 1975 to 2014, among which 6075 were adults (84.5%). Patients: Patients were seen by a multidisciplinary team of experts. The diagnosis was by expert opinion, not by any specific classification/diagnosis criteria. However, 96.8% of them were classified by the International Criteria for Behcet’s Disease (ICBD). Adult BDs were patients who had their first manifestation at the age of 16 or later. Results: Males constituted 56% (male/female ratio 1.3/1). The mean age at onset was 28.3  8.7, mean duration 10.8  8.2, and mean follow-up 5.0  6.3. Oral aphthosis was seen in 97.5%, genital aphthosis 65.7%, skin manifestations 64.6% (pseudofolliculitis 53.2%, erythema nodosum 23.9%), ocular manifestations 58.1% (anterior uveitis 41.1%, posterior uveitis 45%, retinal vasculitis 33.6%, cataract 24.4%), joint manifestations 39.4% (arthralgia 18.9%, monoarthritis 9.1%, oligoarthritis 17.8%, ankyloing spondylitis 2%), gastrointestinal manifestations 7% (gastroduodenitis 2.3%, peptic ulcer 1.2%, diarrhea 2.1%, rectorrhagia 1.0%, abdominal pain-nausea 1.8%), neurological manifestations 10.6% (central 3.7%, peripheral 0.3%, headache 7.6%), vessel involvement (large vessel 1.7% with large vein thrombosis 1.1% and arterial involvement 0.7%, phlebitis 6.6%, superficial phlebitis 2.3%), epididymitis 4.6%, pulmonary manifestations 1% and cardiac manifestations 0.6%. Positive pathergy test was seen in 52.3%, human leukocyte antigen (HLA)-B5 in 54%, HLA-B51 in 48.9%, and high erythrocyte sedimentation rate in 52.8% of patients. By International Study Group (ISG) criteria 77.9% were classified, compared to ICBD revised criteria with 96.9%. The specificity of ISG was 99.2% and ICBD 97.2%.