The development of epidemiological methods for the study of adverse drug reactions is reviewed in connection with the presentation of data obtained by intensive monitoring of 1 000 admissions to a medical paediatric unit. Compared with adults and American children, the patients received fewer drugs and experienced fewer reactions while in hospital. The drug usage pattern was different from that of American paediatric practice and general practice in the United Kingdom. Fifty-one (6%) patients experienced 119 adverse drug reactions. These occurred more frequently in children suffering from serious disorders and in the majority of cases the basic therapy was continued regardless of the severity of the drug side-effects. Treatment was required for the effects of 66 (55%) adverse reactions. It appears that drug monitoring in paediatric practice may be of greater value if surveillance programmes are designed to provide a "therapeutic audit" and extended to include children receiving drugs in the community.