Already in childhood, (a) high general and visceral fatness and poor cardiovascular (CV) fitness are linked with dyslipidemia, elevated blood pressure, and insulin resistance; (b) higher fatness is associated with low amounts of vigorous physical activity (PA) and relatively "low" energy intake; (c) higher CV fitness is more clearly associated with vigorous PA than with moderate PA; (d) higher levels of PA are associated with higher bone density only in youths with relatively high calcium intakes. Physical training in youths has favorable effects on total body and visceral adiposity, bone density, CV fitness, and some risk factors for CV disease and type 2 diabetes. Thus, vigorous PA during childhood may help to prevent some major "adult" health problems. (Contains 8 figures and 3 tables.)