Growth hormone (GH) affects carbohydrate metabolism through direct negative effect on insulin sensitivity and indirectly, via insulin-like growth factor-1 (IGF-1), which exerts positive insulin-mimetic action. The aim of this retrospective study was to evaluate the influence of long-term GH treatment on glucose homeostasis in 118 children with isolated idiopathic GH deficiency (GHD). Based on this analysis we wanted to determine the usefulness of glycated haemoglobin (HbA1c) and parameters derived from the oral glucose tolerance test (OGTT) in the monitoring of disturbed glucose metabolism during GH treatment and to assess the value of IGF-1 in prediction of those changes. Mean duration of GH treatment was 2.5 ± 1.2 years. Data were analysed in the whole group and according to baseline pubertal status. Significant increases in insulin concentrations, both fasting and during the OGTT, accompanied by a significant increase in fasting glucose and unchanged glucose concentrations during the OGTT, were found after the initiation of GH treatment. HbA1c did not change significantly during GH treatment in comparison to baseline values and remained normal, even in patients with impaired fasting glucose (IFG) or impaired glucose tolerance (IGT) found during GH treatment. Changes in glucose metabolism observed after the onset of GH treatment were related to increment in IGF-1 SDS and to GH doses. Significant associations between changes in IGF-1 SDS in the first year of GH treatment and some of the glucose metabolism parameters evaluated after the first, the second and the third year of GH treatment were also confirmed in multiple regression analysis after taking the GH dose into consideration. All cases of IFG and/or IGT detected during GH treatment are reversible after dietary intervention, independently of pubertal status, and do not lead to diabetes mellitus.