Studies to date on the treatment with insulin-like growth factor I (IGF-I) of children with growth hormone (GH) insensitivity syndrome (GHIS) have concentrated principally on the effects of IGF-I therapy on the GH-IGF-I axis and on growth velocity. Little is known, however, about the metabolic status of children with GHIS and the consequences of IGF-I treatment on circulating intermediary metabolites involved in glucose homeostasis. We have studied 5 children with GHIS, aged 4.5–9 years, 3 male and 2 female, before and after 3 months of treatment with recombinant IGF-I, 80 µg/kg s.c. twice a day. The children were short (height SDS –3.8 to –7.3) and growing slowly (height velocity 1.8–3.4 cm/year). All were neurodevelopmentally normal at the time of the study with no history of severe symptomatic hypoglycaemia, in particular during the neonatal period. Before treatment, 4 of the 5 children demonstrated spontaneous hypoglycaemia (blood glucose <2.6 mmol/l), particularly at night, accompanied by substantial hyperketonaemia (range 1.43–4.63 mmol/l) and hyperfattyacidaemia (range 1.11–3.08 mmol/l). After 3 months of IGF-I treatment, the blood glucose concentrations in the diurnal profile were increased, with improvement in spontaneous hypoglycaemia and with increased fasting tolerance. The postprandial insulin-to-glucose ratio was reduced. GHIS is thus associated with asymptomatic nocturnal hypoglycaemia without apparent neurological deficit. Despite functional GH deficiency, brisk counter-regulation to hypoglycaemia occurred as evidenced by hyperfattyacidaemia and hyperketonaemia. Treatment with IGF-I twice a day improved fasting glucose tolerance, diurnal blood glucose concentrations, and postprandial insulin-to-glucose ratios. There was no exacerbation of hypoglycaemia on treatment. We suggest that in severe GH resistance, a protective mechanism exists for the brain from the effects of hypoglycaemia, at least partially mediated by excessive production of counter-regulatory metabolic fuels.