Osteocalcin is a non-collagenous bone matrix protein which is released into the circulation and can be measured by radioimmunoassay. Recent studies indicate that serum osteocalcin concentrations are a marker of bone formation. Because bone demineralisation is a common finding in patients with rheumatoid arthritis (RA) the serum osteocalcin concentrations and, in addition, the serum concentrations of 25-hydroxyvitamin D, parathyroid hormone, and calcitonin were measured in 29 patients with RA and in 30 control subjects. Whereas serum osteocalcin concentrations were similar in patients with RA and in control subjects, serum 25-hydroxyvitamin D concentrations were significantly decreased in patients with RA. Serum concentrations of parathyroid hormone and calcitonin in patients with RA and in control subjects were not statistically different. The normal osteocalcin concentrations in patients with RA suggest a normal rate of bone formation in these patients.