Excess body weight is a risk factor for morbidity and mortality in several studies. The etiology of obesity is, however, poorly understood. We examined the ability of selected behavioral variables to predict weight gain (greater than 6.8 kg) in the San Antonio Heart Study, a population-based study of diabetes and cardiovascular disease in Mexican Americans and non-Hispanic whites. However, we were unable to confirm an effect of dietary variables or level of physical exercise in predicting future obesity in Mexican Americans. To some degree this may be because of the imprecision of the 24-h-dietary-recall technique and the dietary behavior and exercise questionnaires.