The medical records-linkage system of the Mayo Clinic was used to identify cases of idiopathic intracranial hypertension in the 15-year period, 1976 through 1990, among the population of Rochester, Minn. Nine patients (eight women and one man) were identified, corresponding to an average annual age-adjusted incidence rate per 100,000 of 0.9 for the total and 1.6 for the female population. In females aged 15 to 44 years, idiopathic intracranial hypertension occurred at a rate of 3.3 per 100,000 per year; for those defined as obese (body mass index > 26), the rate rose to 7.9. Median follow-up was 2.7 years (range, 5 months to 15 years). Three of 18 eyes developed visual impairment; this was mild in all cases.