INTRODUCTION:Adverse events are relatively common in healthcare, leading to extensive harm to patients and a significant drain on healthcare resources. Identifying the extent, nature and consequences of adverse events is an important step in preventing adverse events and their consequences which is the subject of this study. METHODS:This is a retrospective review of medical records randomly selected from patients admitted to 4 general hospitals, staying more than 24 hours and discharged between April and September 2012. We randomly selected 1200 records and completed the record review for 1162 of these records. Standard forms (RF1 and RF2) were used to review medical records in two stages by nurses and medical doctors. RESULTS:Eighty-five (7.3%) of the 1162 records had an adverse event during the admission; and in 43 (3.7%) of the 1162 records, the patient was admitted to the hospital due to an adverse event that occurred before the admission. Therefore, a total of 128 (11.0%) adverse events occurred in 126 (10.9) records as two patients had more than one adverse event. Forty-four (34.3%) of these 128 adverse events were considered preventable. CONCLUSIONS:This study confirms that adverse events, particularly adverse drug reactions, post-operative infections, bedsore and hospital acquired infections are common and potentially preventable sources of harm to patients in Iranian hospitals.