Abstract Aim: To elucidate the prevalence of Behcet's disease (BD) during the World Health Organization-International League of Associations for Rheumatology Community Oriented Program for Control of Rheumatic Disease study in Iran. The old estimate was 16 per 100,000 inhabitants, but this was just an estimation and a more precise figure was required. Materials and methods: Tehran, the capital of Iran with 1/10th of Iran's population, was selected as the field for the COPCORD study. The population of Iran is of mixed ethnicity (Caucasians 75.4%, Turks 22%, and Semites 2.6%). The same distribution was found in Tehran's general population. Tehran has 22 districts; 50 clusters were randomly selected from them. Interviewers passed an exam by interviewing 20 subjects. The observed agreement was 0.96. The chance expected agreement was 0.531325. The kappa coefficient was 0.919 (standard error: 0.112). The z-score was 8.19. The one tailed P-value was < 0.0001. Rheumatologists were selected from the ‘Rheumatology Subspecialty’, that is, those who were scheduled to become a rheumatology professor in one of Iran's medical schools. Results: The response rate was 75%. From 10,291 subjects interviewed all subjects with positive questionnaires were examined; seven had definitive BD and five probable BD. The prevalence was calculated on the seven definitive cases. It was 68 for 100,000 inhabitants. The confidence interval was 33.5–137. Four patients were female and three were male. All had oral and genital aphthosis, four had ocular lesions, three had pseudo-folliculitis, and one had joint manifestations. Conclusions: The prevalence of BD in Iran is 68 per 100,000 inhabitants, which is the second highest prevalence after Turkey (80–370) in the world, and far behind comes Saudi Arabia with 20, China 14, and Japan 13.4 per 100,000 inhabitants.