BACKGROUND Assuring that medication administration error (MAE) reports are reliable and valid is of great significance for the patient, the hospital, and the nurse. In most hospitals, MAE reporting relies on the nurse who discovers an error to initiate an error report, whether the error was committed by that nurse or someone else. Because of the potential for negative consequences, there may be significant disincentives for the nurse to report the error. This, the first of two articles, describes the results of a large-scale survey designed to assess nurses' perceptions of the reasons why MAE may not be reported. The companion article compares nurses' estimates of the extent to which MAEs are reported with the actual reported medication error rates. METHODS Nurses in 24 acute-care hospitals were surveyed to determine perceptions of reasons why medication errors may not be reported. RESULTS The factor analysis reveals four factors explaining why staff nurses may not report medication errors: fear, disagreement over whether an error occurred, administrative responses to medication errors, and effort required to report MAEs. CONCLUSIONS There are potential changes in both systems and management responses to MAEs that could improve current practice. These changes need to take into account the influences of organizational, professional, and work group culture.