An intensive drug surveillance program has been developed to study the clinical effects of drugs in hospitalized children. This program collects information on drug exposures and the occurrence of adverse clinical events. The 1669 children monitored to date received an average of 7.6 drugs during an average hospital stay of 8.4 days. A group of specified adverse clinical events, whether or not drug attributed, occurred in 45.7% of the patients; drug-attributed events (adverse drug reactions) occurred in 16.8%. Both drug use and reported adverse reactions tended to increase with age, except that newborns received many drugs but had the lowest reported adverse reaction rates. Newborns, however, had the highest rate of adverse events not attributed to drugs, suggesting that perhaps some of these latter events include presently unrecognized adverse drug reactions.