BACKGROUND To evaluate associations between exposure to disinfection byproducts in drinking water and adverse birth outcomes, personal exposure to disinfection byproducts must take into consideration multiple routes of exposure. METHODS We assessed the reproducibility and validity of a questionnaire measuring water consumption, showering and bathing habits, use of chlorine-based products, and swimming pool attendance in 237 pregnant Italian women enrolled between June and December 1999. The questionnaire was completed during the last trimester of pregnancy (preQ) and again a few days after delivery (postQ). Data from postQ were compared with a 7-day diary completed during the last trimester. RESULTS According to postQ, total water intake was 2.6 liters per day, whereas tap water intake was 0.6 liters per day. Intraclass correlation coefficients of postQ compared with preQ were 0.85 for tap water daily intake and 0.77 for duration of showering and bathing. Pearson's correlation coefficients were 0.84 for tap water daily intake, 0.81 for frequency of showering, and 0.94 for bathing. The kappa statistics were 0.76 (95% confidence limits = 0.68, 0.85) for use of domestic chlorine-based products and 0.82 (0.70, 0.94) for indoor swimming. Pearson's coefficients for postQ compared with the diary were 0.78 for tap water daily intake, 0.62 for frequency of showering, and 0.64 for bathing. Compared with the diary, the sensitivity and specificity of postQ in assessing indoor swimming were 75% and 90%, respectively. CONCLUSIONS The questionnaire appears to be a valid and reliable method for assessing exposure to disinfection byproducts in the last trimester of pregnancy.