OBJECTIVES: Learning from clinical data on the subject of safety with regards to patient care in dentistry is still largely in its infancy. Current evidence does not provide epidemiological estimates on adverse events (AEs) associated with dental care. The goal of the dental practice study was to quantify and describe the nature and severity of harm experienced in association with dental care, and to assess for disparities in the prevalence of AEs. METHODS: Through a multistaged sampling procedure, we conducted in-depth retrospective review of patients' dental and medical records. RESULTS: We discovered an AE proportion of 1.4% (95% CI, 1.1% to 1.8). At least two-thirds of the detected AEs were preventable. Eight percent of patients who experienced harm due to a dental treatment presented only to their physician and not to the dentist where they originally received care. CONCLUSIONS: Although most studies of AEs have focused on hospital settings, our results show that they also occur in ambulatory care settings. Extrapolating our data, annually, at least 3.3 million Americans experience harm in relation to outpatient dental care, of which over 2 million may be associated with an error. PRACTICAL IMPLICATIONS: Measurement is foundational in enabling learning and improvement. A critical first step in preventing errors and iatrogenic harm in dentistry is to understand how often these safety incidents occur, what type of incidents occur, and what the consequences are in terms of patient suffering, and cost to the healthcare system.