['To analyze the association of work-related physical activity (WRPA) and leisure-time physical activity (LTPA) with body mass index (BMI) and obesity in the Spanish adult population aged 20 to 60 years.', 'The data were taken from the 1993 Spanish National Health Survey. We analyzed a sample of 12,044 men and women representative of the Spanish population aged 20 to 60 years. BMI and frequency of obesity (BMI > or = 30 kg/m(2)) were obtained from self-reported weight and height. Multiple linear regression and logistic regression models were constructed, adjusting for the main confounding factors. WRPA and LTPA were measured by two questions to classify subjects into four categories of physical activity.', 'Neither mean BMI nor percentage of obesity varied significantly (p > 0.05) by WRPA. Mean BMI was significantly higher (p < 0.01) in those who were inactive in their leisure time (25.90 kg/m(2) in men and 24.43 kg/m(2) in women) than in those who reported vigorous activity (24.42 kg/m(2) and 22.97 kg/m(2) in men and women, respectively). The odds ration (OR) for obesity decreased with increasing level of LTPA in both men (OR of 0.64 for vigorous activity) and women (OR = 0.68), showing a statistically significant dose-response relation in both men (for linear trend, p = 0.0021) and women (p = 0.0245).'] Is work-related physical activity associated with body mass index and obesity?