Twenty-nine hemodialysis patients were studied to evaluate different laboratory measures in assessing muscle protein stores and detecting protein malnutrition. Arm muscle circumference (AMC) was used as a reference for somatic protein stores. AMC correlated with serum complement C3, plasma histidine, isoleucine, leucine, methionine and threonine concentrations as well as with body mass index. The lowest quartile of AMC was found most reliably by measuring plasma methionine, histidine, leucine and isoleucine concentrations. Protein malnutrition was detected in five patients (17%). They had significantly lower serum prealbumin and plasma leucine concentrations than the others. In the follow-up of 3 years every malnourished patient died, three from septic infection. Of the others only seven died, none from infection. The analysis of plasma essential amino acid and serum prealbumin concentrations had an important role in assessing muscle protein stores as well as the protein nutrition status in hemodialysis patients. Patients with malnutrition should be detected in view of their unfavourable prognosis.