A study of prescribing errors committed by physicians that occurred in a tertiary-care teaching hospital is reported. From a total of 289 411 medication orders written during the 1-year study period, 905 prescribing errors were detected and averted, of which 522 (57.7%) were rated as having potential for adverse consequences. The overall detected error rate was 3.13 errors for each 1000 orders written and a rate of 1.81 significant errors per 1000 orders. The error rate (4.01 per 1000 orders) was greatest between 12 pm and 3:59 pm. First-year postgraduate residents were found to have a higher error rate (4.25 per 1000 orders) than other prescriber classes, and obstetrics/gynecology services (3.54 per 1000 orders) and surgery/anesthesia services (3.42 per 1000 orders) had greater error rates than other services. The study results demonstrate the significant risk to patients from medication prescribing errors. Educational, operational, and risk-management activities should include efforts directed at reducing the risk to patients from prescribing errors.