Obesity is a growing problem. In the broadest strokes, it is due to a small positive energy balance that persists over a sufficiently long time. Some forms of obesity develop independent of the type of diet that is eaten, whereas others are dependent on the diet. Among the former are individuals with leptin deficiency or genetic defects in the melanocortin 4 receptor. Most human obesity, however, occurs in the presence of highly palatable foods--fat and calorically sweetened beverages. The increase in obesity in the last 35 years has paralleled the increasing use of high-fructose corn syrup (HFCS), which first appeared just before 1970. Current soft drinks and many other foods are sweetened with this product because it is inexpensive and has useful manufacturing properties. The fructose in HFCS and sugar makes beverages very sweet, and this sweetness may underlie the relation of obesity to soft drink consumption. Fructose consumption has also been related to the metabolic syndrome and to abnormal lipid patterns. This evidence suggests that we should worry about our current level of fructose consumption, which has been increasing steadily for over 200 years and now represents over 10% of the energy intake of some people.