Medication safety in the ambulatory setting is an ongoing challenge, partly driven by the lack of computerized systems that promote safe prescribing. This retrospective cohort study analyzed nearly 4000 computer-generated prescriptions over a 4-week period and found a 12% error rate; 35% were considered potential adverse drug events. The error rates varied for different computerized systems (ranging from 5% to 38%) with omitted information the most common error type (60%). The findings suggest that implementing e-prescribing solutions requires more than simply adopting a computerized system. Careful attention is required to assure safe processes and functionality.