OBJECTIVE: To assess the more prevalent beliefs about body weight and the factors involved in weight changes in the Spanish adult population. DESIGN: A national survey was carried out according to an established protocol on Spanish subjects selected by a multistage procedure following a random route model, which was quota-controlled for several sociodemographic variables. This study was undertaken by the Spanish arm of a pan-European survey and was performed with a validated questionnaire. It contained questions to evaluate some aspects concerning the relationship between obesity, physical activity and health. We also estimated the proportion of self-reported overweight and obesity. SETTING: Spain. SUBJECTS: The sample included 1,000 subjects aged 15 years or older. RESULTS: Eleven per cent of the sample were obese (body mass index, BMI > 30 kg/m-2) and an additional 32% were overweight (BMI > 25 and < 30 kg/m-2). Obesity prevalence was higher among older individuals, those with lower education and socioeconomic levels, and among housewives and retired or unemployed people. Most Spanish people believed that fat intake (51%) and the amount of food consumption (44%) were the major factors involved in weight gain, while physical activity was less mentioned (12%). The method most frequently used to lose weight was diet (9%). Individuals from central and southern regions payed more attention to genetics (20-27%) and physical activity (12-20%) as determinants of weight gain than people living in the north or northwest regions (15-17% and 8-9%, respectively). Normal weight people participated more often in some physical activity during their leisure time. CONCLUSIONS: The Spanish population is not familiar with factors influencing weight gain. Health promotion strategies should emphasize the role of physical activity, especially among older individuals, retired or unemployed subjects, those from lower educational or socioeconomic levels and among people living in the north or northwest of Spain.