To examine whether chlorination of drinking water was associated with cancer of the digestive or other organs, an ecological epidemiological study using nationwide incidence data from the Cancer Registry of Norway was carried out. On two geographical levels (counties and municipalities), both for men and women, chlorination of drinking water was associated with an increased incidence of cancer of the colon and rectum. After adjusting for potential confounding variables, also measured on a geographical basis, the associations were still significant at the county level (adjusted for population density, income, education, fat and fibre intake etc.), but not at the municipality level. The observed associations are weak, chlorination being associated with a 20-40% increase in colorectal cancer rates. Due to inherent methodological limitations in ecological studies like the present one, causal interpretations should be made with great care. Thus, although the results give some support to the hypothesis that drinking water chlorination is associated with colorectal cancer, they do not provide strong evidence of a causal relationship.