Monogenic mutations of the hepatocyte nuclear factor 1 homeobox A maturity onset diabetes of the young (HNF1A-MODY) is characterized by early onset, typically before the age of 25 years. Patients are often not clinically recognized; however, the identification of HNF1A-MODY patients is crucial because they require different antihyperglycemic medical treatment than patients with type 1 or type 2 diabetes mellitus. We describe two adult patients with monogenic diabetes, both identified as HNF1A-MODY, genetically c.815G>A, p.Arg272His and c675delC, p.Ser225Argfs*8, respectively. They were misdiagnosed as having type 1 diabetes mellitus, and consequently, initiating insulin therapy led to hypoglycemia and unstable blood glucose control. Usually, sulfonylureas represent the basis of antidiabetic treatment in patients with HNF1A-MODY; however, all medical personnel involved in diabetes care should be aware of monogenic diabetes mellitus and the possibilities for genetic testing. The patients observed have shown the necessity of the identification and appropriate genetic diagnosis of HNF1A-MODY in order to discontinue insulin therapy and to initiate adjusted diabetes management.