Molecular alterations of the transcription factor hepatocyte nuclear factor 1B (HNF1B) are associated with systemic disease, with kidney disease and maturity-onset diabetes of the young (MODY) as the most characteristic manifestations. Other features comprise pancreatic exocrine insufficiency, liver and biliary anomalies, and genital tract malformations. HNF1B-associated disease is clinically heterogeneous, and therefore the diagnosis is challenging. The authors describe the case of a 19-year-old man with new-onset diabetes after kidney transplantation (NODAT). The kidney disease presented during fetal life as bilateral hyperechogenic kidneys. Renal function progressively deteriorated during childhood, and at the age of 19, the patient was submitted to a living-kidney transplant. Two weeks after transplant, NODAT developed. Given the young age and normal body weight, NODAT was unexpected, and the possibility of HNF1B-associated disease was considered. Screening for mutations in HNF1B was undertaken, and a known mutation was found. As this case highlights, HNF1B-associated disease should be considered when NODAT unexpectedly develops in young kidney transplant recipients with a suggestive renal disease.