1-4% of all visits to emergency rooms are due to adverse drug events (ADEs), but data about preventability and cost due to preventable ADEs in this setting are limited. OBJETIVE: To determine the incidence of ADEs associated with emergency department (ED) visits and subsequent hospital admissions, to asses their preventability and to estimate the costs of preventable ADEs to our institution, a 750-bed university tertiary care center. METHOD: All patients who visited the ED from October 1995 to Mars 1996 were prospectively evaluated. Recorded data included: medical and medication history, laboratory data, serum drug concentrations, ED procedures, intervention therapy, and characteristics and outcome of ADEs. Patients admitted as a result of the ADE were followed to collect information of invasive and non invasive procedures, therapy, length of stay and intensive care. Preventable ADEs were analyzed to identify the associated factors. A cost analysis was performed. RESULTS: From a total of 33,975 visits to ED, 766 (2.25%) were due to ADEs. A total of 336 (43.9%) ADEs were assessed as preventable; these resulted in 121 hospital admissions, 1,575 stays, 121 ICU days and 15 deaths. Inappropiate dose prescribed and inadequate monitoring of drug therapy accounted for 46% and 31% of preventable ADEs; 17% preventable ADEs were due to automedication. The estimated hospital annual cost for all ADEs was $1.42 million and for preventable ADEs was almost $ 1 million. CONCLUSIONS: Many ADES seen in ED patients are preventable and contributed substantially to hospital costs. At a time when much emphasis is put on the limitation of health expenses, here is a domain where expense-cutting could probably be done reasonably easily, while increasing quality of care.