The Wisconsin Heart Health Research Program measured serum lipids and other clinical parameters among residents of forty-six neighboring small communities in central Wisconsin. The purpose of the study was to determine whether distribution of serum lipids, blood pressure or thyroid hormones differed according to the chlorination of the water supply, or to its calcium and magnesium content (hardness). This report examines serum lipid levels in relation to the drinking water characteristics chlorination and hardness. Variables measured on individuals included age, education level, alcohol intake, cigarette smoking, dietary fat and dietary calcium. An analysis of covariance was used to estimate effects of chlorination and hardness on each of the serum lipids, with individual variables included as covariates. Among females, serum cholesterol (SC) levels are significantly higher in chlorinated communities than in nonchlorinated communities. Community SC levels are also higher for males in chlorinated communities, on the average, but differences are smaller and not statistically significant. LDL cholesterol levels follow a similar pattern to that for total SC levels, higher in chlorinated communities for females, but not different for males. On the other hand, HDL cholesterol community means are nearly identical in the chlorinated and nonchlorinated communities for each sex. (Copyright (c) International Epidemiological Association 1990.)