This study analyzed nearly 290,000 medication orders during a 1-year study period to determine the rates and risk factors for prescribing errors. Results indicated an overall error rate of 3.13 errors per 1000 orders written, with the greatest rate seen between noon and 4 pm and among first-year residents. Additional data presented include classes of medication involved in errors, types of errors detected, and frequency of errors by specialty service. The authors conclude that medication errors pose a significant safety risk in teaching hospitals and that system interventions, such as appropriate monitoring of prescribing habits and educational training programs, must be emphasized.