BACKGROUND: Hospital readmission is common among patients with diabetes. Some readmissions, particularly for hypoglycemia and hyperglycemia, may be avoidable with better care transitions and post-discharge management. OBJECTIVE: To ascertain the most common reasons and risk factors for readmission among adults with diabetes, with specific consideration of severe dysglycemia. DESIGN: Retrospective analysis of data from the OptumLabs Data Warehouse, an administrative data set of commercially insured and Medicare Advantage beneficiaries across the U.S. PARTICIPANTS: Adults ≥18 years of age with diabetes, discharged from a hospital between January 1, 2009, and December 31, 2014 (N = 342,186). MAIN MEASURES: Principal diagnoses and risk factors for 30-day unplanned readmissions, subset as being for severe dysglycemia vs. all other causes. KEY RESULTS: We analyzed 594,146 index hospitalizations among adults with diabetes: mean age 68.2 years (SD, 13.0), 52.9% female, and 67.8% white. The all-cause 30-day readmission rate was 10.8%. Heart failure was the most common cause for index hospitalization (5.5%) and readmission (8.9%). Severe dysglycemia accounted for 2.6% of index hospitalizations (48.1% hyperglycemia, 50.4% hypoglycemia, 1.5% unspecified) and 2.5% of readmissions (38.3% hyperglycemia, 61.0% hypoglycemia, 0.7% unspecified). Younger patient age, severe dysglycemia at index or prior hospitalization, and the Diabetes Complications Severity Index (DCSI) were the strongest risk factors predisposing patients to severe dysglycemia vs. other readmissions. Prior episodes of severe dysglycemia and the DCSI were also independent risk factors for other-cause readmissions, irrespective of the cause of the index hospitalization.CONCLUSIONS: Adults with diabetes are hospitalized and readmitted for a wide range of health conditions, and hospitalizations for severe hypoglycemia and hyperglycemia remain common, with high rates of recurrence. Severe dysglycemia is most likely to occur among younger patients with multiple diabetes complications and prior history of such events.