This paper reviews the relationships between physical fitness and body composition and their combined effect on health. After discussing the epidemiologic evidence for a protective effect of physical fitness on the health risks associated with obesity, it describes the Aerobics Center Longitudinal Study, an ongoing observational study that provides data on the relationships between physical fitness and a variety of health outcomes, as well as other studies. Epidemiologic evidence demonstrates a strong protective effect for physical activity and physical fitness on the health risks associated with obesity. Probable mechanisms for the protective effect of physical activity include the reduction and/or reversal of the development of a progressive disease process known as insulin-resistance syndrome, which is closely associated with obesity. A major benefit of physical activity is that it leads to specific reductions in levels of abdominal obesity and induces beneficial metabolic changes that limit the underlying disease process related to fat and dyslipidemia, hypertension, hyperinsulinemia, reduced glucose sensitivity, and ultimately non-insulin dependent diabetes mellitus. Overall, research suggests that the health risks of obesity are largely controlled if a person is physically active and physically fit. (Contains 28 references.) (SM)