Survivors of pediatric hematopoietic stem cell transplantation (HSCT) are known to be at risk of developing endocrine abnormalities, but occurrence of diabetes mellitus (DM) is a relatively recent observation. We present a 17.5-year-old girl with DM after high-dose radio- and chemotherapy followed by allogeneic HSCT for the treatment of acute lymphoblastic leukemia, diagnosed when she was 10 years old. In the posttransplantation period, multiple acute and chronic complications occurred. Among them, we observed graft versus host disease requiring corticosteroid therapy, pancreatitis and some endocrine complications like primary hypothyroidism, growth hormone deficiency and hypergonadotropic hypogonadism. DM with some components of metabolic syndrome-like insulin resistance, high arterial blood pressure and dyslipidemia developed during the first year after HSCT. Five years later, a trend towards increased requirement of insulin with deterioration in metabolic control of DM was observed, despite a normal level of C-peptide and negative diabetes autoantibodies. After the addition of metformin to continuous subcutaneous insulin infusion in the therapy of DM, an improvement in metabolic control was observed. Due to the possible mechanism of insulin resistance which is associated with impaired insulin receptors after HSCT procedure, metformin with insulin appears to be effective in the treatment of this type of diabetes.