The objective of this study was to reanalyze two well-conducted, peer-reviewed epidemiology studies in Iowa and Ontario that reported an increased risk of bladder cancer associated with chlorinated drinking water and trihalomethanes (THMs). Is the risk of cancer from DBPs in chlorinated drinking water greater or less than currently thought? Can specific cancer-causing compounds be identified? Can DBP formation be accurately predicted? This study updates our knowledge of the health effects of disinfection by products in chlorinated drinking water. Includes appendices on CD.