Malnutrition is the most common cause of mortality in the world. It affects underdeveloped as well as industrialized societies, in the latter demonstrating a prevalence in hospitalized patients of between 30 and 50%. Although the prevalence has decreased in recent studies, the problem is still significant among a selected group of patients. The clinical manifestations of malnutrition may be evident on physical examination but alterations in renal function may not show up at the initial exam. Clinical and experimental models of protein-calorie malnutrition have confirmed significant alterations in renal hemodynamics, renal concentration capacity, and renal acid excretion. Children and adults with malnutrition have been shown to have a decreased glomerular filtration rate and renal plasma flow (RPF), as well as a lowered capacity to concentrate the urine and excrete an acid load. Moreover, clinical and experimental models of protein-calorie malnutrition have unravelled the roles of the renin-angiotensin system, renal prostaglandins, and urea production in the renal function changes associated with malnutrition. We have reviewed the most pertinent and recent studies from our and other laboratories which have improved our understanding of renal functional alterations in malnutrition.