Homeostasis of essential amino acids and their transamination derivatives (ketoacids) is disturbed in haemodialysis (HD) patients. In long-term HD patients a hypercatabolic state is often paired with severe anaemia. To understand metabolic regulation mechanisms we measured with an improved fluorescence-HPLC method plasma concentrations of valine (Val), isoleucine (Ile), and leucine (Leu) and their corresponding ketoacids ketoisovaleric acid (KIV), ketomethylvaleric acid (KMV), and ketoisocaproic acid (KIC). The values of 18 modestly anaemic HD patients (group A: Hb greater than 11 g/dl) and of 16 severely anaemic HD patients (group B: Hb less than 8 g/dl) were compared with 19 healthy control persons (100%; significance of patient values vs controls P less than 0.05) and with each other (significantly different at P less than 0.05). Both branched chain amino acids and ketoacids are diminished in HD patients. This disturbance of protein metabolism is intensified with severe anaemia. The decrease of transamination products KIV and KMV parallels that of their corresponding Val and Ile, whereas KIC is reduced out of proportion to Leu. Leu has anabolic function and KIC antiproteolytic effects. Decreased Leu and KIC indicate catabolism. Reduced transamination of Leu and KIC suggests an endogenous protective mechanism against catabolism independent of anaemia. These differences should be considered with supplementation therapy of branched-chain compounds in HD patients.