Overweight, as well as obesity have become mass phenomena with an ever increasing prevalence in most countries all over the world, and are associated with a greater cardiovascular risk and mortality. Weight excess is usually directly related to an additional risk of morbidity and mortality in the general population. In contrast, in patients with end-stage renal disease (ESDR) under haemodialysis a paradox has been reported, showing that an increased body mass index (BMI) was associated with better survival, and vice versa. This paradox relationship, sometimes referred to as "reverse epidemiology" proved to be relatively constant in chronic haemodialysis and to a lesser extent in peritoneal dialysis. Reverse epidemiology is also found in haemodialysis patients for other risk factors, such hypertension, or hypercholesterolaemia. And, likewise, this phenomenon is also observed in other populations of heavy chronic disease suggesting a potential selection bias, such as a real survival benefit for obesity in case of the presence of another heavy chronic disease. The possible causes and mechanisms of reverse epidemiology are reviewed in this article.