PURPOSE: Outpatients with cancer receive complicated medication regimens in the clinic and home. Medication errors in this setting are not well described. We aimed to determine rates and types of medication errors and systems factors associated with error in outpatients with cancer. METHODS: We retrospectively reviewed records from visits to three adult and one pediatric oncology clinic in the Southeast, Southwest, Northeast, and Northwest for medication errors using established methods. Two physicians independently judged whether an error occurred (kappa = 0.65), identified its severity (kappa = 0.76), and listed possible interventions. RESULTS: Of 1,262 adult patient visits involving 10,995 medications, 7.1% (n = 90; 95% CI, 5.7% to 8.6%) were associated with a medication error. Of 117 pediatric visits involving 913 medications, 18.8% (n = 22; 95% CI, 12.5% to 26.9%) were associated with a medication error. Among all visits, 64 of the 112 errors had the potential to cause harm, and 15 errors resulted in injury. There was a range in the rates of chemotherapy errors (0.3 to 5.8 per 100 visits) and home medication errors (0 to 14.5 per 100 visits in children) at different sites. Errors most commonly occurred in administration (56%). Administration errors were often due to confusion over two sets of orders, one written at diagnosis and another adjusted dose on the day of administration. Physician reviewers selected improved communication most often to prevent error. CONCLUSION: Medication error rates are high among adult and pediatric outpatients with cancer. Our findings suggest some practical targets for intervention, including improved communication about medication administration in the clinic and home.