Maturity-onset diabetes of the young type 5 (MODY5) is a rare subtype of MODYs. It caused by mutations of the hepatocyte nuclear factor 1 homeobox b gene (HNF1B). A 21-year-old young woman was admitted to our hospital for severe malnutrition and gastrointestinal symptoms. At age 20, she was diagnosed with type 2 diabetes mellitus and was administered with oral antidiabetic drugs. Soon afterwards, the patient discontinued the medication on her own accord, and then went to the hospital again due to diabetic ketoacidosis. After insulin treatment, diabetic ketoacidosis was cured and blood glucose was controlled satisfactorily. But intractable nausea, vomiting and persistent weight loss was stubborn. Further examination revealed that the patient had hypokalemia and hard rectification hypomagnesemia. Genetic testing revealed about 1.85Mb heterozygous fragment deletion on chromosome 17 and deletion of exons 1-9 of HNF1B heterozygosity missing was approved. Finally, the patient was diagnosed MODY 5 with HNF1B heterozygosity missing based on 17q12 recurrent deletion syndrome. Key words: Maturity-onset diabetes of the young 5 (MODY5), Hepatocyte nuclear factor 1 homeobox b gene (HNF1B), 17q12 Recurrent deletion syndrome