BACKGROUND Maturity onset diabetes of the young (MODY) usually presents in patients under the age of 25 years and is an autosomal dominant condition associated with mutations in the hepatocyte nuclear factor 1 alpha gene, glucokinase gene, or hepatocyte nuclear factor 4 alpha gene. This report is of a series of 4 cases from Poland of MODY type 5 associated with mutations in the hepatocyte nuclear factor 1 beta (HNF1B) gene, including a 13-year-old boy and adult men aged 33, 34, and 35 years. CASE REPORT Three cases were diagnosed late, in patients in their mid-thirties. In two patients, the initial presentation was symptomatic diabetes complicated by ketoacidosis and hyperglycemic hyperosmolar state. Renal cysts were found in all patients, and pancreatic hypoplasia in 3 patients. All patients except 1 were negative for autoantibodies; 1 presented with hypomagnesemia. Insulin therapy was instituted in all cases. The combination of family history, imaging study results, and biochemical characteristics led to the decision to perform genetic analysis, which was conducted in 2 cases at diagnosis, and in the 2 remaining patients at 1 month and 2 years after diagnosis, respectively. Follow-up data revealed hypomagnesemia and/or hypermagnesuria in all patients. CONCLUSIONS We present 3 young men over 25 years and 1 boy with HNF1B-MODY. Although rare, autosomal dominant gene associations should be considered in young patients with diabetes who present with renal/pancreatic anomalies and low serum magnesium. Unusual presentation and the presence of autoantibodies should not eliminate the possibility of a HNF1B defect.