We identified all residents of Rochester, Minnesota, who sought medical assistance for the first time in 1985 because of symptomatic osteoarthritis of the hip or knee that was unrelated to a specific disease. Of these residents, 98 (59 women and 39 men) had 122 symptomatic joints (95 knees and 27 hips), for age- and sex-adjusted incidence rates of 205 new patients and 255 newly affected joints per 100,000 person-years. The incidence of osteoarthritis of the hip was greater in women than in men, whereas the sex ratio for occurrence of osteoarthritis of the knee approached unity. Rates at both sites increased steadily with aging in men but plateaued after the menopause in women; similar patterns were seen whether or not grade 1 disease was included. Projected nationally, these first population-based data indicate that as many as approximately half a million new symptomatic cases of idiopathic osteoarthritis of the knee and hip may arise annually in the white population of the United States. With increasing longevity, osteoarthritis may exact an even greater toll in the future.