The objective of this study was to evaluate the reliability of a self-administered postal questionnaire on the use of food supplements. The study was carried out in subjects representative of an Italian adult population. Eight thousand eight hundred twenty-three subjects received the questionnaire; 1723 subjects completed it of which 102 twice (baseline and 1-month re-administration). The latter 204 questionnaires were used to test reliability using Cohen’s kappa statistic (k) and the intra-class correlation coefficient (ICC) for categorical and quantitative variables, respectively. Subjects’ characteristics such as sociodemographic and physical data, lifestyles, dietary habits, and most health characteristics showed very good agreement (ICC or k 1.00 - 0.55) between questionnaires, with the exception of answers about the consumption of some medicines (k 0.37 - 0.40). The reliability concerning the use of food supplements was satisfactory on the whole (k 0.69) and fairly satisfactory for different categories of food supplements (k 0.83 - 0.41). With regard to additional information about users of food supplements, the reliability of responses was fairly satisfactory on the whole (k 0.93 - 0.41), with some exceptions. The concordance/correlation coefficient values generally showed that the questionnaire is fairly reliable over the entire sample for collecting information on the use of food supplements.