Both deficiency and excess of GH are related to disturbances in calcium metabolism. Bone Gla protein (BGP) is the only specific marker of bone turnover identified in peripheral blood. We, therefore, determined plasma BGP in 21 adult GH-deficient patients treated with biosynthetic human GH in a double blind cross-over study. We also examined 9 patients with acromegaly before and after surgery. The GH-deficient patients had normal initial plasma BGP concentrations, whereas the acromegalic patients had highly significantly increased concentrations (P less than 0.001). During treatment with human GH, plasma BGP (and other nonspecific biochemical markers of bone turnover) increased significantly (P less than 0.001). During placebo treatment plasma BGP showed baseline values. In the acromegalic patients a significant decrease in plasma BGP concentrations was seen 1 week after surgery. The present study suggests that plasma BGP is a useful biochemical marker of the effect of treatment of both GH deficiency and GH excess/disorders.