The aim of this study was to evaluate the usefulness of serum osteocalcin (OC) levels in multiple myeloma (MM) in order to assess its significance and activity, and to predict its course. Serum OC was measurement in 117 patients with MM and 116 healthy controls matched for age and sex. Serum OC levels were weakly correlated with Karnofsky index (r=0.22; p<0.03). Lowest OC levels were observed when lytic bone lesions increased (p<0.05). There was no relationship between serum OC levels and vertebral crush fractures, serum calcium concentrations or stage of MM, neither was there any relationship between initial serum OC levels and survival. Progression of the disease was associated with a clear fall of serum OC in 61.5% of the "progressive" patients, versus 41% of the persisting "stabilized" cases. Serum OC level was strongly correlated with bone formation (p = 0.005), but not with bone resorption. Serum OC level is a sensitive marker of osteoblast activity, but a poor marker of the severity of MM. We do not consider it as a marker of MM activity or prognosis.