In 260 (3.7%) of 7,017 medical patients, an adverse drug reaction either caused or strongly influenced admission to the hospital, according to the admitting physician. The five drugs that were most commonly implicated as causes of hospital admission were digoxin (42 patients), aspirin (24), prednisone (15), warfarin sodium (9), and guanethidine sulfate (5); 113 other drugs were implicated. The five most common adverse events were digitalis intoxication (46 patients), gastrointestinal bleeding (24), hypoglycemia (13), rash (13), and gastritis (12). Since a major segment of the outpatient population that is served by the wards monitored in this study consume large amounts of many drugs, adverse drug reactions requiring hospitalization for any one drug are uncommon.