It has been suggested that there are two distinct types of involutional osteoporosis in women. Loss of ovarian function causes spine and wrist fractures in middle-aged women whilst a decline in renal endocrine function and bone formation is responsible for fractures, especially of the hip, in elderly women. We have investigated calcium regulation in post-menopausal women with hip fractures in their seventh decade together with non-osteoporotic controls of similar age. The major findings were reduced secretion of calcitonin (P less than 0.01) and 1,25 dihydroxy-vitamin D (P less than 0.025) in the osteoporotics as compared with controls. No differences in plasma levels of calcium, parathyroid hormone and 25 hydroxyvitamin D were observed. Our results show some similarities with those reported in both proposed types of osteoporosis, suggesting considerable overlap between them. Secondary hyperparathyroidism does not appear to be a usual feature of osteoporotic women with hip fracture up to age 75 years. Our hormonal findings are more in keeping with marked oestrogen deficiency, suggesting that loss of ovarian function is a major determinant of the osteoporosis in these women.