OBJECTIVE: To determine the incidence and prevalence of sporadic inclusion body myositis (sIBM) and polymyositis (PM) in a population-based study. METHODS: Charts of patients with myositis in Olmsted County, Minnesota, USA, from 1981 to 2000 were reviewed. RESULTS: For sIBM, the age- and sex-adjusted incidence rates per 100,000 were 0.79 (95% confidence interval = 0.24-1.35), and for PM, 0.41 (95% CI 0.08-0.73). The age- and sex-adjusted prevalence rates per 100,000 were 7.06 (95% CI 0.87-13.24) for sIBM and 3.45 (95% CI 0.00-7.35) for PM. CONCLUSION: The incidence and prevalence rates for sIBM are higher than previously reported.