Some studies indicate that chlorination by-products in drinking water may contribute slightly to breast cancer risk. This ecologic study describes the association between total trihalomethane levels in publicly supplied water and the incidence of female invasive breast cancer. We included 71 North Carolina water suppliers serving at least 10,000 customers in the summer of 1995 as the units of analysis. We estimated incidence rates using 6,462 cases who were either white or black and between 35 and 84 years old and were linked by zip codes to the water supplier. We treated ecologic measurements of age, income, education, urban status, and race as potential confounders. Total trihalomethane levels were not associated materially with breast cancer risk, adjusting for potential confounders. The rate ratio for 80.0 parts per billion (ppb) or more vs less than 40.0 ppb total trihalomethanes was 1.1 [95% confidence interval (CI) = 0.9-1.2]. When stratified by race, the observed association for the aforementioned total trihalomethane category was not very different in black women (rate ratio = 1.2; 95% CI = 0.8-1.8) than in white women (rate ratio = 1.1; 95% CI = 0.9-1.3). These ecologic data are compatible with trihalomethanes in drinking water being either unrelated or weakly related to breast cancer risk.