This report summarizes the assessment made of an accidental overexposure of radiotherapy patients that occurred at the San Juan de Dios Hospital in San Jose, Costa Rica, in August and September 1996. The assessment was carried out by an Expert Team convened by the IAEA in July 1997 at the request of the Government of Costa Rica. The findings of the Expert Team's assessment are presented in two parts: (1) an evaluation of the doses to patients by analysing the treatment records and physical measurements; and (2) a medical examination of patients, together with the autopsy findings for those who died. Thirty cross-sectional images are reproduced that show the dose reconstructions made for some of the patients. The purpose of the report is to foster information exchange with a view to preventing similar accidents in the future.