Quantify the impact of hypoglycemia on outcomes of hospitalized diabetic patients and determine how variations in the definition of hypoglycemia affect outcomes. This study used an EMR database of inpatient and ED encounters for adults with diabetes treated at 70 hospitals during 2000-2006. Patients presenting to the ED and subsequently admitted as inpatients were considered as one encounter. Encounters with hypoglycemia were categorized into 3 mutually exclusive groups: 1) encounters due to hypoglycemia required a primary inpatient diagnosis or any ED diagnosis of hypoglycemia (ICD-9-CM code 251.0, 251.1, or 251.2); 2) encounters in which patients presented with hypoglycemia (ie, first lab blood glucose [BG] during the first 24 hours in the hospital < 70 mg/dL); and 3) encounters in which patients developed hypoglycemia only after the first 24 hours in the hospital. Outcomes were total charges, length of stay, discharge to a skilled nursing facility (SNF), and inpatient mortality. Encounter types 2 and 3 were each compared with encounters without hypoglycemia (ie, all BG values =70 mg/dL). Comparisons were adjusted for potential confounders (e.g., reasons for hospitalization, comorbidities, propensity score for hypoglycemia) using multivariate regression models. Hypoglycemia was rarely used as a primary diagnosis. Of 519,317 encounters (n=215,922 diabetic patients), 732 (0.14%) had hypoglycemia as the reason for the encounter, and most (91%) were ED only without admission. However, in 8,971 encounters (1.73%), patients presented with hypoglycemia. In an additional 19,487 encounters (3.75%), patients developed hypoglycemia. All outcomes for patients who developed hypoglycemia were significantly different than those for patients without hypoglycemia. However, only charges and odds of discharge to a SNF were significantly associated with presentation with hypoglycemia. The effect of hypoglycemia on outcomes depends on when hypoglycemia is defined and is more pronounced in patients who developed hypoglycemia than in patients who presented with hypoglycemia.