This study investigated potential causes of obesity by examining diet and lifestyle factors. The data from the 2008 Korea National Health and Nutrition Survey were statistically analyzed to determine the relative importance of causes of obesity. Because the factors affecting obesity for males and females were significantly different, binary choice logistic models of the male and female subjects were built and estimated separately. Our results show that stress, the irregularity of eating breakfast, and frequency of eating out had the three greatest impacts on male obesity, respectively, and stress, employment status, and age had the greatest impacts on female obesity, in that order.