Objective: We used MEDMARX®, the national medication error-reporting program, to compare medication errors reported by critical access hospitals (CAHs) to those reported by non-Federal, general community hospitals.Methods: We used the availability of pharmacist support to represent the structure and process of medication use and within-cluster resampling to account for the correlation of error reports within hospitals.Results: CAHs with 15 or fewer hours of pharmacist support per week were significantly less likely to report “near miss” errors—a characteristic of high-reliability organizations—than general community hospitals with 24-hour pharmacist support. Conclusion: The severity of voluntarily reported medication errors is associated with the structure and process of medication use as indicated by the availability of pharmacist support. MEDMARX is a potential data source for patient safety organizations (PSO). PSOs must consider varying structure and process within reporting organizations and account for the correlation of data within clusters.