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Insulin edema is a poorly understood complication of insulin therapy. It has been reported in patients with both type 1 and 2 diabetes mellitus and typically occurs in patients with newly diagnosed or poorly controlled diabetes mellitus either after initiation or intensification of insulin therapy. A 20-year-old man presented with anorexia, polydipsia, and weight loss. Serum glucose on admission was 824 mg/dL (45.8 mmol/L) and hemoglobin A1c was >14.0. Additional workup was notable for positive anti-IA2 antibodies and low C-peptide of 0.5 ng/mL (1.1-4.4 ng/mL). He was diagnosed with type 1 diabetes mellitus and was started on insulin therapy with glargine and lispro. Within 4 days after insulin initiation, he developed bilateral leg swelling and reported a 25-pound (11.3-kg) weight gain over the next 10 days. After excluding other systemic causes of edema such as heart failure, renal failure, and liver failure, a diagnosis of insulin edema was made. Insulin glargine was switched to insulin degludec. Complete resolution of edema occurred within 3 days of switching the insulins. Insulin edema is a diagnosis of exclusion. Insulin's role in renal sodium handling, vasodilation, and increased vascular permeability have been postulated as possible mechanisms. Clinicians should be aware of this rare complication. [ABSTRACT FROM AUTHOR]