We report 34 patients (aged 5-18 years) with acute (n = 26) or chronic (n = 1) leukemia, non-Hodgkin's lymphoma (n = 3) or severe aplastic anemia (n = 4) evaluated for pancreatic beta-cell function 9 months to 10.2 years after autologous (n = 19) or allogeneic (n = 15) BMT. Before BMT, all patients received cytotoxic drugs, combined with total body irradiation (TBI) in 24 cases or thoracoabdominal irradiation (TAI) in 4 children. Patients were investigated for fasting blood glucose (FBG), HbA1C, anti-insulin (IAA) and islet cell antibodies (ICA), first-phase insulin response (FPIR) and insulinemia/glycemia (I/G) ratio on i.v. glucose tolerance test (GTT) and C-peptide response after glucagon 1 mg i.v. Results were compared with those obtained in 21 age- and sex-matched controls. None of the patients or controls had IAA and/or ICA. FBG and HbA1C were normal in all children. In the patients, glycemia on i.v. GTT was similar to controls whereas insulin levels I/G ratio and FPIR were significantly higher in patients than in controls, as well as C-peptide levels. We divided the patients on the basis of the radiotherapy into group I with TBI (n = 18), group II with TAI (n = 4) and group III who were not irradiated (n = 4). The I/G ratio, FPIR on i.v. GTT and C-peptide response were significantly higher in group I compared with the other two groups and controls. The increased insulin and C-peptide levels in our patients with normal glycemia might be interpreted as a state of insulin resistance, more evident in patients who received TBL.