Our objective was to estimate the prevalence of mortality and serious morbidity for office-based deep sedation and general anesthesia (DS/GA) for dentistry in Ontario from 1996 to 2015. Data were collected retrospectively in 2 phases. Phase I involved the review of incidents, and phase II involved a survey of DS/GA providers. In phase I, cases involving serious injury or death for dentistry under DS/GA, sourced from the Office of the Chief Coroner of Ontario and from the Royal College of Dental Surgeons of Ontario (RCDSO), were reviewed. Phase II involved a survey of all RCDSO-registered providers of DS/GA in which they were asked to estimate the number of DS/GAs administered in 2015 and the number of years in practice since 1996. Clinician data were pooled to establish an overall number of DS/GAs administered in dental offices in Ontario from 1996 to 2015. Prevalence was calculated using phase I (numerator) and phase II (denominator) findings. The estimated prevalence of mortality in the 20-year period from 1996 to 2015 was 3 deaths in 3,742,068 cases, with an adjusted mortality rate of 0.8 deaths per 1 million cases. The estimated prevalence of serious morbidity was 1 injury in 3,742,068 cases, which adjusts to a serious morbidity rate of 0.25 per 1 million cases. The mortality rate found in this study was slightly lower than those published by earlier studies conducted in Ontario. The risk of serious morbidity was found to be low and similar to other studies investigating morbidity in office-based dental anesthesia.