1. Three cases of a benign osteoblastic lesion of bone are described. An outstanding feature of each was the hyperostosis of adjacent bones or synovitis in an adjacent joint.

2. The clinical, radiological and histological features resembled osteoid osteomata more than benign osteoblastoma in each case.

3. The significance of this observation is questioned in relation to the pathogenesis of osteoid osteoma.