Given the concern for the impact of adverse drug reactions on hospital morbidity and costs, this study recorded and classified all such events from a multihospital registry. The most commonly implicated medications included digoxin, aspirin, prednisone, and warfarin, with the most common events being digitalis toxicity and gastrointestinal bleeding. The authors conclude that, although adverse events from a single drug requiring hospitalization are uncommon, adverse drug reactions as a category represent a major source of hospitalizations.