BACKGROUND Adverse drug events (ADEs) continue to be of concern to all health professionals. Even serious ADEs are underreported in all patient-care environments. OBJECTIVE To discover the incidence and the best detection methods and preventability for ADEs at Al Qassimi Hospital, a 360-bed facility in the United Arab Emirates. METHODS During the first and fourth quarters of 2003, data collection for ADEs was limited to spontaneous reporting. During the second and third quarters, active monitoring for ADEs took place in the adult, pediatric medical, and intensive care wards. ADEs were assessed for causality using the Naranjo algorithm and for severity and preventability. The incidence of ADEs was calculated and the detection methods were compared. RESULTS The incidence of ADEs detected through surveillance was significantly higher (p < 0.001) than for ADEs reported spontaneously for both inpatients (3.592 vs 0.068/100 patient days) and outpatients (0.299 vs 0.022/100 patient visits). Most ADEs were judged to be of mild to moderate severity. About 56% of ADEs were judged definite or probable and, of these, 13.8% were consistently judged preventable. The most prevalent drugs implicated were central nervous system (23.6%), antiinfective (17.1%), and cardiovascular (16.5%) agents. The best ADE detection method was using physicians' notes. CONCLUSIONS Active surveillance for ADEs, with the aid of ADE trigger alerts, yields a significantly higher number of reports than spontaneous reporting. Such surveillance is useful in identifying areas where improvements in the safe use of drugs can be made.