Researchers are only gradually becoming aware of the gravity of the risk that overweight and obesity pose for children's health. In this article Stephen Daniels documents the heavy toll that the obesity epidemic is taking on the health of the nation's children. He discusses both the immediate risks associated with childhood obesity and the longer-term risk that obese children and adolescents will become obese adults and suffer other health problems as a result. Daniels notes that many obesity-related health conditions once thought applicable only to adults are now being seen in children and with increasing frequency. Examples include high blood pressure, early symptoms of hardening of the arteries, type 2 diabetes, nonalcoholic fatty liver disease, polycystic ovary disorder, and disordered breathing during sleep. He systematically surveys the body's systems, showing how obesity in adulthood can damage each and how childhood obesity exacerbates the damage. He explains that obesity can harm the cardiovascular system and that being overweight during childhood can accelerate the development of heart disease. The processes that lead to a heart attack or stroke start in childhood and often take decades to progress to the point of overt disease. Obesity in childhood, adolescence, and young adulthood may accelerate these processes. Daniels shows how much the same generalization applies to other obesity-related disorders--metabolic, digestive, respiratory, skeletal, and psychosocial--that are appearing in children either for the first time or with greater severity or prevalence. Daniels notes that the possibility has even been raised that the increasing prevalence and severity of childhood obesity may reverse the modern era's steady increase in life expectancy, with today's youth on average living less healthy and ultimately shorter lives than their parents--the first such reversal in lifespan in modern history. Such a possibility, he concludes, makes obesity in children an issue of utmost public health concern. (Contains 1 table and 96 notes.)