Serum bone Gla protein (BGP), a sensitive and specific biochemical marker of bone formation, was measured in 66 children deficient in GH before and after 3, 6, 9, and 12 months of treatment with biosynthetic human GH. Initial serum BGP levels were significantly lower than those of normal age-matched children. After 3 months of treatment, the mean serum BGP level had increased to normal. There was no relation between baseline serum BGP and the growth response, but the GH-induced increment in serum BGP levels at 3 months was highly significantly related to growth response after 12 months of therapy. The study shows that serum BGP could be a helpful biochemical marker in prediction of the growth response to long term GH therapy.