The assessment of past exposure is a prerequisite to all epidemiological studies on drinking water and cancer. In this study the past exposure assessment of drinking water carcinogenicity was done in terms of the drinking water mutagenicity estimated from historical water parameters and compared with the methods used previously in past exposure assessments in studies on drinking water and cancer. The method was applied in 56 municipalities in Finland. The comparison of different methods in past exposure assessment suggests some advantages for the method presented as it allows a quantitative exposure assessment based on historical information on drinking water mutagenicity. Nevertheless the relevance of the method is with respect to the role of mutagenicity in carcinogenicity and the water type in question.