Three hundred eighty-one total hip arthroplasties were performed on residents of Olmsted County, Minnesota, during the period from 1969 to 1980, for a rate (adjusted for sex and age) of 44.6 per 100,000 person-years. Rates rose with age, were higher for women than men, and were higher among urban than rural residents of the county. If we assume that the Olmsted County experience is medically optimal and apply it to the 1980 United States population, we calculate a national requirement of over 100,000 total hip arthroplasties per year, well above the current actual figure. If this calculated number of total hip arthroplasties were actually performed each year, over 1.4 million hospital days would be required, and direct medical costs would probably exceed $1 billion annually.