Objective: Patients using U-500 regular insulin are severely insulin resistant, requiring high doses of insulin. It has been observed that a patient's insulin requirements may dramatically decrease during hospitalization. This study sought to systematically investigate this phenomenon. Methods: We performed a retrospective chart review of patients with U-500 insulin outpatient regimens who were admitted to the San Antonio Military Medical Center over a 5-year period. Each patient's outpatient total daily dose (TDD) of insulin was compared to the average inpatient TDD. The outpatient estimated average glucose (eAG) was calculated from the glycated hemoglobin (HbA1c) and compared to the average inpatient glucose. Results: There were 27 patients with a total of 62 separate admissions. The average age was 64.4 years, with a mean body mass index of 38.9 kg/m2 and eAG of 203 mg/dL (HbA1c, 8.7%, 71.6 mmol/mol). All patients were converted from U-500 to U-100 upon admission. The average inpatient TDD of insulin was 91 units, versus 337 units as outpatients (P<.001). Overall, 89% of patients received ≤50% of their outpatient TDD. The average inpatient glucose was slightly higher than the outpatient eAG, 234 mg/dL versus 203 mg/dL (P =.003).Conclusion: U-500 insulin is prone to errors in the hospital setting, so conversion to U-100 insulin is a preferred option. Despite a significant reduction in insulin TDD, these patients had clinically similar glucose levels. Therefore, patients taking U-500 insulin as an outpatient can be converted to a U-100 basal-bolus regimen with at least a 50% reduction of their outpatient TDD.