BACKGROUND Growth hormone (GH) promotes longitudinal growth in children with growth hormone deficiency. Serum bone alkaline phosphatase (BALP) has been considered to be a good marker of bone formation with narrow circadian variation. The aim of the study was to determine the effect of GH treatment on serum BALP levels in children with idiopathic GH deficiency. METHODS A total of 26 prepubertal children (10 girls) with GH deficiency in mean age 9.06+/-1.24 yrs were enrolled in the study. They were treated with daily sc injections of GH (0.5 IU/kg/week). Serum BALP levels were measured using enzyme immunoassay (Alkaphase-B kit, Metra Biosystems) at baseline, 3 and 6 months of GH treatment. RESULTS A significant increase in serum BALP levels after 3 months of GH treatment was observed. No significant differences between BALP levels after 3 and 6 months of GH therapy were stated. Baseline serum BALP levels correlated positively with the height velocity during the 3 months period before the GH treatment initiation (r=0.63, p<0.05). The BALP increase after 3 months of treatment correlated with height velocity in that period (r=0.69, p<0.05). The change in BALP after 3 months of GH treatment correlated with the improvement in the height after 3 and 6 months (r=0.76, p<0.05) as well as after 12 months of GH treatment (r=0.77, p<0.05). CONCLUSIONS We conclude that serum BALP appears to be a good early predictor of the growth responses to GH therapy in GH-deficient children.