BACKGROUND: Some children born small for gestational age (SGA) experience supra-physiological insulin-like growth factor-I (IGF-I) concentrations during GH treatment. However, measurements of total IGF-I concentrations may not reflect the bioactive fraction of IGF-I which reaches the IGF-I receptor at target organs. We examined endogenous IGF-bioactivity using an IGF-I kinase receptor activation (KIRA) assay that measures the ability of IGF-I to activate the IGF-IR in vitro. AIM: To compare responses of bioactive IGF and total IGF-I concentrations in short GH treated SGA children in the North European Small for Gestational Age Study (NESGAS). RESULTS: Bioactive IGF increased with age in healthy pre-pubertal children (n=94). SGA children had low-normal bioactive IGF levels at baseline (-0.12 (1.8 SD), increasing significantly after one year of high-dose GH treatment to 1.1 (1.4) SD, p<0.01. Following high-dose GH, 68% (n=65) of SGA children had a total IGF-I concentration >2SD (mean IGF-I 2.8 SDS), whereas only 15% (n=15) had levels of bioactive IGF slightly above normal reference values. At baseline, bioactive IGF (SDS) was significantly correlated to height (SDS) (r=0.29, p=0.005), in contrast to IGF-I (SDS) (r=0.17, p=0.10). IGF-I (SDS) was inversely correlated to delta height (SDS) after one year of high-dose GH treatment (r=-0.22, p=0.02). CONCLUSION: In contrast to total IGF-I concentrations, bioactive IGF stayed within the normal reference ranges for most SGA children during the first year of GH treatment.