Maturity Onset Diabetes of the Young (MODY) is a type of diabetes that results from mutations in 13 known genes that play a role in the development and maturation of pancreatic beta cells. It represents 5% of all individuals diagnosed with diabetes before the age of 45 years and it is misdiagnosed as type 1 or type 2 diabetes in 80% of the cases. We present the case of a 21-year old female, initially diagnosed with type 1 diabetes when she was 19 years, and glycemic dyscontrol despite adequate use of long-acting insulin. She had family history of diabetes and persisting glycosuria. Due to high suspicions of MODY diabetes we requested an oral glucose tolerance test (basal 130 mg/dL, 2 hours post-glucose load 240 mg/dL) and serum concentrations of C-peptide (1.83 ng/mL) that confirmed our diagnosis. The patient improved after treatment with sulfonylurea with discontinuation of insulin treatment. Prompt identification of MODY allows patients to have effective and safe treatments and prevent the development of premature complications; in addition, its identification allows genetic counseling and can guide the management of other first-degree relatives who also suffer from it.