Background: In-hospital insulin administration is associated with many medication errors, but the frequency and reasons for insulin administration errors are poorly described. To document types and frequency of errors related to insulin administration, an examination of 4 units was conducted. Methods: Using snapshot methodology, 4 non-intensive care unit (ICU) areas (medicine, cardiology, transplant, and surgery) were examined in an observational, prospective manner for 4 weeks. Each patient on insulin on the first day was followed for 7 days. Definitions and error categories were defined prior to data collection. Error types and numbers were collected and quantified on per-day or per-patient basis. Results: A total of 116 patient audit periods covering a total of 378 inpatient hospital days were examined. Inpatient insulin regimens on day 1 included correctional insulin only (51.7% of cases), neutral protamine Hagedorn ([NPH] 12%), and glargine (28.4%). A total of 199 administration errors occurred at a rate of 1.72 errors/patient-period and 0.53 errors/patient day. Missing documentation of doses (15.5% of all patients) and insulin being held without an order (25% of patients) were the most frequently occurring events. Other errors include transcription (7.5%), timing errors (22.7%), and lack of documentation of physician notification of hypoglycemia (12.6%). Conclusions: Errors associated with insulin in the hospital are common and reveal a number of system errors that should be addressed. These data provide a foundation for future performance improvement.