A total of 521 patients consecutively admitted to an acute geriatric unit were kept under surveillance by one observer during their stay. All drugs given to them and the occurrence of adverse events were recorded. One hundred and seventeen adverse drug reactions occurred in 94 patients representing 18.8% of the 500 patients receiving drugs. Thirteen of these reactions were considered severe. Each patient received an average of 6.1 drugs, not necessarily simultaneously. Altogether 212 different drug preparations were used. Diamorphine and insulin had the highest adverse reaction rates, diamorphine having the highest risk of a severe adverse reaction. Antibiotics and diuretics caused the most adverse reactions, and were by far the most commonly prescribed drugs (26.5% of the sample). Of the patients receiving eight or more drugs, 41% suffered an adverse drug reaction.