Abstract Disclosure: Z. Saeed: None. C. Labib: None. M. Trabulsi: None. M. Varatharajah: None. Introduction: Ketosis-prone diabetes (KPD) also known as Flatbush diabetes, atypical diabetes and type 1.5 diabetes was first reported in the 1990s in African Americans in the Flatbush neighborhood of Brooklyn, New York. Its exact incidence is still unknown; however, it predominantly affects African American and Hispanic population and is less prevalent in Asians and Caucasians. It presents as diabetic ketoacidosis, a hyperglycemic emergency, in patients with no prior history of diabetes. In addition, it is usually seen in middle-aged, obese patients with a strong family history of Type 2 diabetes mellitus. Here we briefly describe the case of a lean 62-year-old man who presented to the hospital with HHS/DKA and was diagnosed with ketosis-prone diabetes. Case Presentation: 62-year-old man (BMI 21.37 Index) with a past medical history of alcoholic cirrhosis, Crohn’s disease and no personal or family history of diabetes presented to the hospital with complaint of nausea, polyuria and polydipsia. He was found to have hyperosmolar hyperglycemic state with diabetic ketoacidosis. He had a blood glucose of 833 mg/dL (70-110 mg/dL), anion gap of 14, sodium 111 mmol/L (136-145 mmol/L), potassium 7.0 mmol/L (3.5-5.1 mmol/L), creatinine 5.7mg/dL (0.6-1.3 mg/dL), pH 7.26 L (7.35-7.45 pH units), bicarbonate 16 mmol/L (22-34 mmol/L). He was started on an Insulin drip and later transitioned to subcutaneous long and short acting insulin after closure of anion gap. His Hemoglobin A1c was found to be 12.8% (4.8-6.1% A1C, Cpeptide 4.32 ng/mL (0.48-5.05 ng/mL), GAD65 antibody was negative. Four months prior to his hospital admission, his Hemoglobin A1C was 4.9%. Patient was discharged from the hospital on Insulin NPH/Regular 70/30 twice a day and sliding scale with meals. He was followed outpatient in clinic. Within a month of discharge he began to have episodes of symptomatic hypoglycemia, lowest recorded blood sugar was 49. His Insulin NPH/Regular was gradually tapered; he did not require the sliding scale at all. Four months after the episode of HHS/DKA, his repeat hemoglobin A1C was 5.7%. Case Discussion: Despite being known for around three decades now, the exact mechanism of ketosis prone diabetes is still poorly understood. It is thought to be due to insulin resistance as well as severe deficits in insulin secretion. It is classified further based on beta cell function and autoimmunity. In patients with preserved beta cell function and absence of autoimmunity, like our patient, with aggressive management insulin can eventually be discontinued. However, these patients remain at risk of developing diabetes later in life. Our case aims to shed light on the ubiquity of this disease so that physicians can diagnose it promptly and treat accordingly. Presentation: Thursday, June 15, 2023