The authors compared seven radiologic indices of hip osteoarthritis to establish which provided the best definition of the disease for epidemiologic purposes. Hip joints were assessed from intravenous urograms taken in a British hospital between 1982 and 1987 in 1,315 men aged 60-75 years. The indices examined were an overall qualitative grading of osteoarthritis, four measures of joint space, the maximum thickness of subchondral sclerosis, and the size of the largest osteophyte. Minimal joint space (i.e., the shortest distance between the femoral head margin and the acetabulum) was the index most strongly associated with other radiologic features of osteoarthritis. Among a subset of 759 men who answered a questionnaire about symptoms, the overall qualitative grading, minimal joint space, and thickness of subchondral sclerosis were the radiologic indices most predictive of hip pain. Within- and between-observer repeatability were tested in a subset of 50 subjects. Measures of joint space were more reproducible than other indices. These data suggest that, at least in men, minimal joint space is the best radiologic criterion of hip osteoarthritis for use in epidemiologic studies.