The number of obese people worldwide has escalated recently, revealing a complex picture of significant variations among nations and different profiles among adults and children, regions, and occupations. The commonly held causes of obesity-overeating and inactivity-do not explain the current obesity epidemic. There is evidence of a general decrease in food consumption by humans and a significant decline in their overall levels of physical activity. There is also more evidence to indicate that the body's natural weight-control mechanisms are not functioning properly in obesity. Because the obesity epidemic occurred relatively quickly, it has been suggested that environmental causes instead of genetic factors maybe largely responsible. What has, up to now, been overlooked is that the earth's environment has changed significantly during the last few decades because of the exponential production and usage of synthetic organic and inorganic chemicals. Many of these chemicals are better known for causing weight loss at high levels of exposure but much lower concentrations of these same chemicals have powerful weight-promoting actions. This property has already been widely exploited commercially to produce growth hormones that fatten livestock and pharmaceuticals that induce weight gain in grossly underweight patients. This paper presents a hypothesis that the current level of human exposure to these chemicals may have damaged many of the body's natural weight-control mechanisms. Furthermore, it is posited here that these effects, together with a wide range of additional, possibly synergistic, factors may play a significant role in the worldwide obesity epidemic.