The Bristol Stool Form Scale (BS) is validated as a surrogate measure for gastrointestinal transit time, and has been also applied to an evaluation of stool consistency. Subjects compare their stool with the BS chart consisting of seven categories and choose one score which has a nearest analog form. The BS has been widely used in the studies to evaluate changes in stool consistency after dietary intervention using probiotics and/or prebiotics. However, there is a lack of evidence in the validity of BS to yield consistent and reproducible estimates of changes in stool consistency over time. In order to obtain a closer estimate of its change, it would be preferred to use a more directly observable and objective parameter or combination of parameters.