Computerized provider order entry (CPOE) reduces overall medication error rates, but this effect may depend on effective clinical decision support systems (CDSS). Whereas most previous longitudinal studies of outpatient CPOE have been performed in academic settings, this study examined community-based primary care providers, who actually write the bulk of ambulatory prescriptions. Rates of prescribing errors immediately following CPOE implementation were low, with approximately 6 errors per 100 prescriptions. These rates were sustained at 1 year following implementation. A previous AHRQ WebM&M perspective described the importance of thoughtful application of CDSS for medication prescribing.