To assess the occurrence of premature ovarian failure, the age-specific incidence rates of natural menopause were determined for a cohort of 1858 women born between 1928 and 1932. These women were identified as Rochester, Minnesota residents in 1950 and were followed for date and type of menopause. A total of nine experienced natural menopause before age 40 years, which represents a 1% risk of natural menopause to age 40. The annual incidence rates of natural menopause per 100,000 person-years were ten for ages 15 to 29 and 76 for ages 30 to 39. In the age group 40 to 44, the incidence of natural menopause increased greatly to 881 per 100,000 person-years at risk.