Using questionnaire is a common, efficient and cheap way to collect data in population-based epidemiological studies. However, there is a general trend of declining response rates leading to a higher risk for bias affecting the validity of results. Use of anonymous questionnaires and/or other ways of distributions, e.g. via schools instead of postal, may increase response rates, but can also lead to other types of systematic errors. This study indicates that results from cross-sectional questionnaire studies differ depending on distribution method and whether the survey is anonymous or not. Also the official purpose of the survey may affect the outcome. When studying well-known risk-factors and health effects, potential methodological problems like reporting bias must be considered in cross sectional studies.