The potential reproductive effects of long-term, low-dose exposure to chloroform have received little attention despite the known, acute toxicity of high exposures and the widespread occurrence of low concentrations of chloroform in drinking water. The association of waterborne chloroform was studied with low birth weight, prematurity, and intrauterine growth retardation. Cases were not mutually exclusive, but each outcome was analyzed independently. Exposures to chloroform and other trihalomethanes were ecological variables based on maternal residence and 1987 municipal water survey. After adjustment for maternal age, parity, adequacy of prenatal care, marital status , education, and maternal smoking by multiple logistic regression, residence in municipalities where chloroform concentration in water supplies was 10 ug/liter was associated with an increased risk for intrauterine growth retardation. The major limitations of this study involve the ascertainment and classification of exposures to trihalomethanes, including such issues as the imprecision of using aggregate municipal measures for classifying exposures at the level of the individual, the potential misclassification due to residential mobility, and the fluctuation of trihalomethane levels.