This research tests hypotheses concerning the relationship between drinking water characteristics and the cardiovascular disease risk factors blood pressure and serum lipids, as well as thyroid function. The study used a cross-sectional survey design to examine the characteristics of the cardiovascular disease risk factors in residents of forty-six small communities in central Wisconsin. Twenty of the communities chlorinated the water supply, and twenty-six did not. Within each of the chlorination groups, approximately half the communities had hard water and the other half had soft drinking water. This report examines the relationship of chlorination to serum lipid levels in the forty-six communities. Serum cholesterol levels were significantly higher in chlorinated communities for females. Levels of serum cholesterol were also higher in chlorinated communities for males, but the difference was not as great, nor was it statistically significant. LDL levels were higher in chlorinated communities, but did not reach statistical significance for either sex. Levels of high-density lipoprotein cholesterol were related to calcium level of the drinking water within the hardness categories, but only in the chlorinated communities. Thus chlorine and calcium level in the drinking water may interact in some way which affects levels of HDL cholesterol. 24 refs., 1 fig., 2 tabs. (ERA citation 13:012615)