For both hemodialysis and CAPD patients nutrition has been linked to mortality. Protein calorie malnutrition is present in 20 to 40% of CAPD patients. The normalized protein catabolic rate (NPCR), has been proposed as a useful measure of dietary protein intake and ultimately nutrition. However, the NPCR value has not been consistently predictive of outcome in CAPD patients. We have performed a cross sectional study on 147 clinically stable CAPD patients, who had a mean dialysis duration 22 months, to evaluate the relationship between the NPCR and conventional markers of nutrition. The NPCR was significantly correlated with normalized models of dialysis adequacy including KT/V (urea), total weekly creatinine clearance and the dialysis index. A significant negative correlation was found between individual anthropometric measures and the NPCR. Using a composite nutritional index to nutritionally categorize our population we found a significantly higher NPCR value in the severely malnourished group. The unadjusted protein catabolic rate (PCR) was significantly correlated with individual nutritional measures and was significantly greater in the well-nourished group. The NPCR, obtained by dividing the PCR by body weight (itself a nutritional measure), is lowest in well-nourished or obese patients, and thus as a marker of nutrition may be flawed. The PCR has nutritional relevance, however, adjusting its value to take into account patient size will require prospective evaluation of the influence of small solute removal on body composition.