The 88 counties of Ohio were classified as either ground water or surface water counties based on the source of the drinking water used by a majority of the county residents included in the 1963 U.S. Public Health Service Inventory of Municipal Water Facilities. Average cancer mortality rates for surface and ground water counties were compared using analysis of covariance. Mortality rates for stomach, bladder, and all malignant neoplasms were higher for white males in counties served by surface water supplies than in counties served by ground water supplies. Mortality rates for stomach neoplasms were higher for white females in surface water counties. These differences in mortality rates were not attributable to other factors known to be associated with cancer death rates including urbanization, median income, population size, manufacturing activity, and agriculture-forestry-fishery activity.