The purpose of this study was to determine the prevalence of errors in the medication system of a pediatric teaching hospital. Error was defined broadly to capture all deviations in the process from medication order through administration of the dose. The long-term goal was to provide direction to efforts to error-proof the system. The sample was 3,312 medication orders written during 669 patient-days for which a total of 11,978 doses were passed. Errors were categorized as intercepted errors (intercepted through the normal processes of the medication system) or administration errors (errors that involve the patient with or without adverse sequelae). Errors were also categorized as errors in primary activities (e.g., prescribing or preparing the medication for administration) or supporting activity (e.g., transferring the order to another record). A total of 784 errors were identified; 98% were intercepted and 2% were administration errors. More errors (71%) occurred in supporting activities than in primary activities. Medication systems are complex processes. Errors are imbedded in the medication system and are typically intercepted before patients are involved. Intercepting errors involves additional work that adds to an already cumbersome process. Error proofing will be different for errors in primary activities and for supporting activities.