Chlorine, commonly used to disinfect drinking water, produces by-products known from animal studies to be carcinogenic and mutagenic. Most epidemiological studies into the possible association between chlorination by-products in drinking water and cancer have been ecological in nature, or have relied on case-control designs based on death certificates. Interpretation of results arising from these studies is limited. Individual levels of toxicant exposure and many potential confounders and effect modifiers are unable to be accounted for in the analyses. At best, these studies generate hypotheses that require more definitive investigation. Misclassification of individuals based on inaccurate assessment of the level of exposure is probable. The few analytic studies able to overcome or minimise these problems suggest a clear link between exposure to chlorinated drinking water and the development of urinary bladder cancer. They also suggest a possible link with rectal cancer. However, these studies have classified subjects by exposure to chlorinated drinking water, rather than to levels of chlorine and its by-products in drinking water. To date, the link between levels of chlorine and its by-products in water, levels of consumption and cancer has not been made. Information on the levels of chlorine and some by-products is available in many water jurisdictions in Australia. Further, epidemiological methods can be employed to quantify water consumption. Case-control studies linking these parameters would help us to understand the magnitude of the risk to human populations and provide a basis to investigate mechanisms for risk reduction.