The relationship between peak growth hormone (GH), insulin-like growth factor I (IGF-I), IGF-I binding protein 3 (IGFBP-3) and IGFBP-3 protease activity was studied in 28 children and adolescents undergoing investigation of pituitary function 0.4-14.2 years after cranial or craniospinal irradiation for the treatment of CNS tumours distant from the hypothalamic-pituitary axis (n = 16) or prophylaxis against CNS leukaemia (n = 12). Seven out of 15 patients with GH deficiency (GHD) (defined as a peak GH concentration <7.5 ng/ml in a stimulation test) had IGF-I <-2 standard deviation score (SDS). None of the 28 patients had serum IGFBP-3 concentrations measured by radioimmunoassay (RIA) <-1.5 SDS with no difference between those with and without GHD. IGFBP-3 concentrations measured by RIA were strongly correlated to IGFBP-3 band density on Western ligand blot (WLB) (r = 0.71; p < 0.0001). IGFBP-3 protease activity was negatively correlated to IGFBP-3 by RIA (r = -0.55; p < 0.01) and to IGFBP-3 by WLB (r = -0.51; p < 0.01). Twenty-two patients had normal IGFBP-3 protease activity (<30% of the activity in pregnancy serum) indicating that serum IGFBP-3 protease activity does not account for the normal levels of IGFBP-3 in RIA. Low serum IGF-I but normal IGFBP-3 concentrations and in the majority normal IGFBP-3 protease activity was found in patients in the years after CNS irradiation. Neither serum IGF-I nor IGFBP-3 can be used as a reliable index of the development of radiation-induced GHD.