Levels of serum osteocalcin (OC) are increased in diseases with high bone turnover. We determined OC levels in (1) 15 patients with definite rheumatoid arthritis (RA) in early stages according to Steinbrocker's functional class FC I-II, (2) 40 patients at advanced stages (FC III-IV) and (3) 17 patients with late RA (onset at age of 65 or more). Sixty-two healthy volunteers, divided into 3 subgroups corresponding to the patients, and 19 patients with primary fibromyalgia syndrome (FMS) served as controls. All patients were included in a short term as well as a longitudinal study over one year. Mean OC levels were significantly elevated in patients with late onset RA compared with healthy controls (p = 0.037), while the OC values in early RA FC I-II and advanced RA FC III-IV did not differ significantly from the corresponding control group and the patients with FMS. The late RA group showed a positive correlation between OC and the erythrocyte sedimentation rate (ESR) (r = 0.641, p = 0.007) with a significant decrease of OC (p less than 0.01) as well as ESR (p = 0.047) over one year. We conclude increased OC levels correlate with disease activity in older patients with active RA, suggesting impaired bone turnover. This finding supports the picture of heterogeneity in RA with more late onset patients displaying "high bone turnover."