The purpose of this dissertation was to determine how phosphorus (P) and protein affects the health of patients with chronic renal failure (CRF). Regular assessment of nutritional status of patients treating with dialysis is vital for optimal nutrition. In the dissertation, we focused on protein-energy malnutrition (PEM), which is one of the strongest predictors of morbidity and mortality. Therefore, the recommended daily protein and energy intake is very high, 1.2–1.4 g protein/kg body mass (BM)/day and 30–35 kcal/kg BM/day. We then focused on P and its significance in CRF. The gradual decline in renal function involves a disturbance of normal P homeostasis, as the kidneys lose the ability to excrete P through urine. There is a close relationship between dietary protein and P intake, as it is found in high protein foods and food containing additives. Therefore, patients should be careful whilst trying to reach their daily protein intake, so that they don't exceed their recommended P intake of 800–1000 mg/day. We also shouldn't ignore the difference between organic and inorganic sources of P. In the case of CRF, the assistance of a clinical dietitian is important in order to acquaint the patient with an appropriate diet that will have the most beneficial effect on his or her health. In the last part, we describe different approaches on how to deal with the problem. The phosphorus food pyramid is also a very useful and easy to understand tool that shows patients which foods they should preferably consume. In general it is recommended to consume high protein foods of high biological value, that have a low P content. For ease of presentation, an example of a one-day diet is included at the end.