UNLABELLED MEDICAL HISTORY AND CLINICAL FINDINGS: We report on a 17-year-old boy with elevated blood glucose levels, elevated liver enzymes and obesity (BMI 32.3 kg/m2). Clinical examination showed acanthosis nigricans and a vitiligo. The rest of the physical examination was without pathological findings. INVESTIGATIONS The HbA1c value was 8.6 % (71 mmol/mol), and postprandial C-peptide showed a maximum level of 1.3 nmol/l. The type 1 diabetes-associated autoantibodies against protein tyrosine phosphatase IA-2 and zinc-transporter-8 were positive, while autoantibodies to glutamic acid decarboxylase and insulin were negative. There was no ketonuria. Ultrasound showed steatohepatitis. TREATMENT AND COURSE Under therapy with metformin up to 2×1 g, blood glucose levels and liver enzymes normalized after a few weeks. After two months, the HbA1c value was 6.0 % (42.1 mmol/mol), and a weight loss of 5 kg was recorded. CONCLUSION In obese adolescent patients with diabetes, a clear classification right from the beginning is not always possible. Characteristic findings of type 1 and type 2 diabetes may be present simultaneously. In the presented patient, monotherapy with metformin was sufficient in the first year. Close monitoring is essential to detect the transition to insulin dependence in time.