Epidemiological studies to evaluate the association between environmental exposure to risk factors and negative health effects often use population level aggregated data to measure exposure, but do not consider personal characteristics that may affect the degree of exposure at the individual level. The aim of this study was to evaluate the validity and reproducibility of a questionnaire specifically designed to measure individual oral, inhalation and transdermal exposure to disinfection byproducts in drinking water. Reproducibility of the questionnaire was evaluated by administering the questionnaire twice, in different time periods, to the same subjects. On the other hand, validity was tested by comparing responses to the questions with information contained in diaries filled out by the subjects daily over a period of one week (gold standard). Questionnaire reproducibilty and validity were both found to be very good, as shown by high Spearman's correlaton coefficients, Intraclass correlation coefficients (ICC), kappa concordance coefficient, sensibility and specificity. The use of a questionnaire such as the one evaluated in this study may be a valuable aid when conductin epidemiological studies to assess individual environmental exposure to risk factors found in drinking water and to explain the effect of such factors on human health.