Purpose To determine the incidence of Graves' ophthalmopathy. Methods A population-based cohort of all Olmsted County, Minnesota, residents who had ophthalmopathy associated with autoimmune thyroid disease between Jan. 1, 1976, and Dec. 31, 1990, was identified through the medical diagnostic index of the Mayo Clinic and the Rochester Epidemiology Project. Results One hundred twenty incident patients were identified, of whom 103 (85.8%) were women (P = .00001; normal relative deviate test). The overall age-adjusted incidence rate for women was 16.0 cases per 100,000 population per year, whereas the rate for men was 2.9 cases per 100,000 population per year (standardized rate ratio, 5.5; 95% confidence interval, 3.3 to 9.3). The distribution of incidence rates by five-year age groups included peak incidence rates in the age groups 40 to 44 years and 60 to 64 years in women, and 45 to 49 years and 65 to 69 years in men. Conclusions Incidence rates for Graves' ophthalmopathy exhibited an apparent bimodal peak for both men and women, although the peaks for men occurred approximately five years after those for women. No explanation for these trends was apparent from the data collected.