Beresford, S. A. A. (Clinical Epidemiology and Health Services Research Unit, V.A. Medical Center, Durham, NC 27705). Cancer incidence and reuse of drinking water. Am J Epidemiol 1983; 117: 258–68.Several studies in the United States have shown an association between the use of river water as a drinking source and cancer mortality, especially gastrointestinal and urinary tract cancer. In the United Kingdom, many lowland rivers are used both as a drinking water source and as a means of transporting domestic and industrial wastes to the sea. The contribution of rivers to the drinking water supply of London residents has been increasing gradually over the last 50 years. This study of 14 boroughs in the south London area examines cancer incidence using registry data for the period 1968–1974, to assess possible risks associated with water reuse. Socioeconomic characteristics of the boroughs were obtained from the 1971 census. Positive associations were found between the average percentage of domestic sewage effluent in the water supplied to a borough and the incidence of stomach cancer and of urinary cancer in females. These associations were reduced when social factors and variations in borough size were taken into account. In spite of the interpretational problems common in aggregate studies, this study, when considered with the results of the US studies, provides some consistent epidemiologic evidence of a small health risk associated with the reuse of drinking water. The size of the risk in the United Kingdom needs to be confirmed by a definitive study on individuals.