Blood flow to bone was measured using the 18F clearance method described by Wootton et al. (1976) in osteomalacia (nine cases) and primary hyperparathyroidism (eight cases). Bone blood flow was elevated above normal in the osteomalacia group and was normal in the hyperparathyroid group (range 3.6%-6.8% blood volume/min). It is suggested that bone blood flow is linked with the osteoblastic response of bone, and remains normal in cases of hyperparathyroidism when no clinical signs of bone involvement are present.