We have reviewed whether the complications of obesity can be attributed to low levels of physical activity and fitness, or if fatness per se has adverse effects on health. In both observational and interventional studies, fatness and inactivity seem to be independent risk factors. Abdominal obesity is a predictor of cardiovascular disease, which is independent of low fitness level, and obesity affects other risk factors adversely than inactivity does. However, physical activity does reduce the risk of type 2 diabetes and heart disease. It is concluded that both inactivity and obesity have an adverse effect on health.