The serious health hazards posed by adverse drug reactions have long been recognised, but the application of epidemiological principles to their studies is only a recent development. A total of 3160 patients admitted into Medical Unit I, General Hospital, Singapore were kept under surveillance for adverse reactions to drugs for six months in 1972. Fifty three of them (1.7%) were admitted with adverse drug reactions as the sole reason for admission. There were 25 males and 28 females. There were two deaths and ten life-threatening reactions, and hypersensitivity was the most common type of reaction encountered. Among the chief offending drugs were Chinese herbal medicines, digoxin, corticosteroids, antibiotics, phenothiazines, and hypoglycaemic agents. All doctors especially the general practitioners must be fully aware of these hazards so that they can help prevent un- necessary morbidity and mortality and also to avoid taxing heavily on the already over-burdened hospital service in a developing country.