Serum osteocalcin was measured by radio-immunoassay in 56 patients with rheumatoid arthritis (RA), and in 50 controls. Mean serum osteocalcin levels were significantly higher in patients with RA. There was a positive correlation between osteocalcin and fasting mucopolysaccharide/creatinine ratio in both sexes, and between osteocalcin and fasting hydroxyproline/creatinine ratio in women. Serum alkaline phosphatase activity, a less specific marker for bone formation, was also increased in RA, and there was a positive correlation with osteocalcin in both sexes. These data suggest that overall bone turnover is increased in RA and that serum osteocalcin may provide additional information for the evaluation of bone metabolism in this disease.