Abnormalities in nutritional status of peritoneal dialysis (PD) patients include too high body mass (overweight, obesity), too low body mass (underweight, starvation) or changes in body composition (malnutrition) without or with normal body weight. In vivo neutron activation analysis is considered the reference gold standard for the determination of protein malnourishment in end-stage renal disease patients, but body mass index (BMI) is the most frequently used parameter in nutritional assessment surveys. The association between BMI and outcome of PD patients is controversial, but so-called obesity paradox (the higher BMI the longer survival) remains frequently reported. The use of metabolic syndrome with high BMI as a crucial component is not more predictable in the prognosis of outcome in PD patients than using separately each risk factor of metabolic syndrome. Underweight/starvation is univocally underlined as associated with morbidity and mortality, but prevalence of severe undernutrition is decreasing over last decades, at least in well developed countries. PD patients may also present features of malnutrition without decreased body mass or even with increased body weight. It mainly concerns to deficiencies of vitamins, minerals and trace elements. Serum albumin concentration has serious limitations as a marker of nutritional status, because is influenced by volemic status and inflammation. Nutritional interventions in undernourished patients (oral, intestinal or intravenous feeding, amino acid peritoneal solution, supplementation of vitamins and trace elements) may correct deficiencies, but their influence on PD patients survival remains unclear.