Plasma protein and amino acid concentrations have been reported to be abnormal in patients with chronic renal failure, whether on conservative or regular dialysis treatment. These abnormalities may be related to impaired protein and amino acid metabolism associated with uremia, to dietary deficiencies of calories and proteins or to amino acid and protein losses due to peritoneal dialysis or hemodialysis. Plasma free amino acid concentrations were evaluated in 17 children undergoing hemodialysis (HD) and 13 children treated by continuous ambulatory peritoneal dialysis (CAPD). Plasma levels of free amino acids showed a reduction of EAA and of the ratio EAA/NEAA. There were some abnormalities in plasma amino acid concentrations; these included decreased levels of valine, threonine, lysine, serine, tyrosine, arginine, alpha-ABA. Aspartate, glycine, citrulline, and, only in HD, cystine and methionine were increased. Plasma protein and amino acid concentrations in CAPD patients are similar to those found in HD patients; thus they result poorly affected by different dialysis techniques and the uremic state itself seems to play a more decisive role.