We defined the clinical features and radiographic coexistence of osteoarthritis and osteoporosis in fifty women with primary coxarthrosis and fifty age-matched women (average age, 69 +/- 5 years) with idiopathic osteoporosis. The patients with osteoarthritis had undergone total hip replacement. The diagnosis of primary osteoarthritis was established by clinical, radiographic, and histological criteria, and the diagnosis of osteoporosis was confirmed by histomorphometric analysis of specimens taken at iliac-bone biopsy. In the arthritic patients, spinal osteoporosis was identified by radiographic evidence of compression fractures, kyphosis, and scoliosis. Femoral osteoporosis was demonstrated by the index of Singh et al. and the femoral canal-shaft ratio. In the osteoporotic patients coxarthrosis was measured radiographically by the scale of Kellgren and Lawrence. On the average, the osteoporotic patients were twelve kilograms lighter than the osteoarthritic patients. Fifty-eight per cent of the osteoporotic and 18 per cent of the osteoarthritic patients had a scoliotic curve of at least 10 degrees. The prevalence of osteoarthritis of the hip in the osteoporotic women was 4 per cent, and the prevalence of compression fractures in the arthritic women was 6 per cent, which was approximately one-quarter of the expected incidence. These results show that: (1) osteoporosis does not protect against the development of coxarthrosis, (2) coxarthrosis is a negative risk factor for osteoporotic compression fractures, and (3) scoliosis and an ectomorphic habitus are clinical markers that identify a risk for osteoporosis.(ABSTRACT TRUNCATED AT 250 WORDS)