Rheumatoid arthritis and primary hyperparathyroidism have been diagnosed in three patients. This is likely to represent the coincidence of two common diseases but the clinical features are discussed with particular reference to the presence and withdrawal of raised levels of circulating parathyroid hormone (PTH). It is argued that raised levels of PTH potentiate the effects of the rheumatoid disease on bone and joints. Other musculoskeletal features of hyperparathyroidism and the actions of PTH on bone are reviewed.