Background.Antimicrobial stewardship programs (ASPs) decrease unnecessary antimicrobial use, decrease antimicrobial resistance, and improve patient outcomes. The effectiveness of a prior approval system—that is, the requirement that approval be obtained from ASP practitioners before certain antimicrobials can be used—depends on the accuracy of the patient data communicated from the primary service.Objectives.To determine the incidence of inaccurate communication of patient data during ASP interactions, describe examples of inaccurate communications, and identify risk factors for inaccurate communication.Design.We used a retrospective cohort design. We evaluated the communicated patient data for clinically important inaccuracies, using the patients' medical records as the gold standard.Setting.A tertiary care medical center that has a prior approval system for restricted antimicrobials.Patients.Inpatients discussed in telephone ASP interactions.Intervention.Observational study.Results.Of telephone calls requesting prior approval from ASP practitioners, 39% (95% confidence interval [CI], 31%-48%) contained an inaccuracy in at least 1 type of patient data (eg, current antimicrobial therapy); the incidence varied widely between data types. Examples of inaccuracies are given to demonstrate their clinical relevance. In multivariable analysis, inaccurate communications were more common for telephone calls from surgical services (versus calls from nonsurgical services: odds ratio, 2.1 [95% CI, 1.1-3.9]) and for calls received by Infectious Diseases fellows (versus pharmacists: odds ratio, 2.0 [95% CI, 1.1-3.8]).Conclusions.A high proportion of ASP calls requesting prior approval included patient data inaccuracies, which have the potential to affect the prescribing of antimicrobials. Although risk factors were identified, these communication errors were common across the different types of ASP interactions. Inaccurate communications may compromise the utility of ASPs that use a prior approval system for optimizing antimicrobial use.