The aim of this study was to investigate cases of accidental ingestion or aspiration occurring at Tokyo Dental College Chiba Hospital over the last 4 years in order to determine how the incidence of such events could be reduced. Forty cases of accidents occurring at our hospital over a 4-year period commencing in 2008 (representing 27% of the total number of accidents) included accidental ingestion in 39 patients and aspiration in one. Most of these accidents occurred during the removal or placement of restorations or prosthetics, and the ingested objects were mostly crowns and inlays. Accidental ingestion or aspiration occurred more frequently in the right molar region and when procedures were conducted by practitioners with less than 1 to 7 years of experience, and especially 1 to 3 years only. A higher rate of such accidents was observed in male patients in their 50s to 70s. The conventional safety procedures developed by the Medical Risk Management Team should be adhered to wherever possible. Furthermore, we propose the following measures based on the present results: accident prevention training for students and clinical trainees; improvement of the in-hospital manual; personal coaching for those breaching the guidelines of the safety manual; and raising awareness of the need for greater care in preventing incidents of accidental ingestion or aspiration at the Tokyo Dental College Chiba Hospital.