Iowa towns of 1000-10,000 population, whose water came solely from wells of over 500 feet (152 meters) in depth and was not treated by a process that would remove radioactivity, were identified. Age-adjusted, sex-specific, cancer incidence rates were determined for these towns for the years 1969-1978 (excluding 1972) and related to the mean level of radium-226 in the municipal water supply. Incidence rates of cancers of the lung and bladder among males and of cancers of the breast and lung among females were higher in towns with a radium-226 level in the water supply exceeding 5.0 pCi/l. A gradient of increasing cancer incidence associated with rising radioactivity level for three time periods was also seen for lung cancer among males. The associations between cancer incidence and radioactivity of water supply could not be explained by smoking patterns, water treatment factors, other water quality measurements, or known socio-demographic features.