The serum levels of osteocalcin (OC), a sensitive and specific biochemical marker of bone formation, were measured in 36 asthmatic children before and after intravenous administration of glucocorticoid (GC), methylprednisolone (1 mg/kg/6 h). A statistically significant (p < 0.005) decrease in OC level (13.5-2.7 microg/L) after the completion of 1-day treatment. In patients who received 2-day treatment, the OC levels further declined and reached about 20% of the pretreatment levels. Blood samples collected from patients who received 3-day treatment showed serum OC had declined to an even lower level after the completion of therapy (11.5-1.8 microg/L). An inverse relationship was found between the OC concentration and the duration of GC therapy. GC therapy induced a minor, significant decrease of serum alkaline phosphatase level but did not influence serum calcium level. A tendency toward decrease of serum phosphate level was also noted during GC treatment. An acute effect of GC therapy on the suppression of osteoblasts was biochemically revealed by the finding of reduced serum OC levels. The results suggest that early change in serum OC may be a useful indicator for patients at high risk of bone loss.