Adverse drug events in hospitalized patients lead to increased morbidity, mortality and costs. Early detection of adverse drug events could aid in the prevention of these adverse outcomes. A cost-effective system for the early detection of adverse drug events should focus on high risk patients. A study was set up with the primary aim to identify characteristics that are associated with the development of adverse drug events (ADEs) in hospitalized patients. ADE reports were gathered from physicians and nurses (spontaneous reports) and from patients after intensive ward interviews by hospital pharmacists. All patients admitted to the internal medicine wards of two Dutch hospitals, during a two month period, were included. The following characteristics were analyzed for their potential relationship to the occurrence of ADEs: age (categorized), gender, number of drugs prescribed during hospital stay, types of drugs used and changes in drug use on admission. Age was found to be inversely associated with the development of ADEs (OR 0.36, CI 0.21-0.61 for age category > 80 years; OR 0.56; CI 0.31-1.02 for age category 75-80 years and OR 0.69; CI 0.42-1.11 for age category 60-74 years). Furthermore, statistically significant associations were found for the number of drugs prescribed per hospitalized patient (for the class of 4-6 drugs per patient OR 2.61, CI 1.32-5.18), for newly prescribed drugs (OR 6.65, CI 2.63-16.81) and for the cessation of drugs on hospital admission (OR 1.50, CI 1.02-2.20). The use of gastrointestinal drugs (OR 2.13, CI 1.32-3.45), central nervous system drugs (OR 1.66, CI 1.07-2.57) and antibiotics (OR 2.44, CI 1.65-3.60) were associated with the development of ADEs, when compared to all other drugs taken by the patients. In this study, the most important risk factors are the number of drugs used per patient and the starting of a new drug during hospitalization. As most hospitalized patients start new drug therapies while in hospital, this seems an inappropriate focus. However, careful monitoring of patients using more than 7 drugs at a time may be possible in a cost-effective system for the early detection of ADEs.