BACKGROUND: Several epidemiological studies suggested an association between vitamin/mineral dietary supplement use and cancer risk. However, characteristics of supplement users may themselves be related to cancer risk, and therefore could confound such etiological studies. Very little is known about the characteristics of French supplement users.AIM OF THE STUDY: To identify cancer-related behaviors and dietary characteristics of vitamin/mineral supplement users in the E3N cohort of French women.METHODS: Data on supplement use and cancer-related and socio-demographic characteristics were collected by self-administered questionnaires completed by 83,058 women, 67,229 of whom also completed a food frequency questionnaire. Supplement users were compared to non-users by unconditional logistic regression.RESULTS: Vitamin/mineral supplement users were significantly older and leaner (odds ratio [OR] for BMI >/= 30 vs. <18.5 kg/m(2) = 0.35, 95% confidence interval [CI] 0.31-0.39), were less often current smokers, had a higher level of education and had more leisure physical activity. They used more phytooestrogen supplements (OR=3.95, 95 % CI 3.69-4.23), had more often a family history of breast cancer and had more often undergone cancer-screening. Users tended to have a healthier diet: less alcohol, more vegetables, fruit, dairy products, fish and soups. They had higher dietary intakes for most micro-nutrients, fiber and omega3 fatty acids, lower fat intake and either similar or lower prevalence of inadequate dietary intake for all relevant nutrients except magnesium.CONCLUSIONS: To avoid major confounding, the lifestyle characteristics of supplement users should be considered in studies investigating the association between supplement use and cancer risk.