In phenylketonuria (L-phenylalanine hydroxylase deficiency, McKusick 26160) the blood concentrations of phenylalanine metabolites, of which one end-product of the transamination chain is phenyllactic acid, may be more relevant for the CNS and thus a better indicator of acute neurotoxicity than the blood concentrations of phenylalanine (Langenbeck et al., 1988). The relation of plasma phenylpyruvic acid to phenylalanine has already been studied (Langenbeck et al., 1988), but not of plasma phenyllactic acid to phenylalanine. We determined plasma phenyllactic acid in 54 phenylketonuric patients, who according to the established criteria are characterized by phenylalanine in serum at 1200μmolL⁻¹ or greater and tyrosine below 280μmolL⁻¹ on a normal diet. The age of the patients ranged from 8 months to 33 years. All samples were collected in a 'semi-fasting' state at noon. The organic acids were analysed according to the method of Rumsby et al. (1987). The identity of the phenyllactic acid peak was confirmed by cochromatography with an authentic standard on the Aminex column as well as on a reversed-phase column (10μ-Bondapak C18, Bischoff, D-7250 Leonberg, FRG; buffer potassium dihydrogen phosphate 20 g L⁻¹ adjusted to pH 2.3 with o-phosphoric acid). The lowest concentration for phenyllactic acid to be detected was 2μmolL⁻¹. Determination of phenylalanine in serum was done by ion-exchange chromatography, in the same sample. According to the Spearman rank correlation procedure we found a correlation between the concentrations of the two compounds (rs = 0.843 [n = 54], p < 0.0001). Scattering of phenyllactic acid concentrations increases with higher phenylalanine concentrations. In patients managed according to the criterion of serum phenylalanine the differences in the concentrations of phenyllactic acid should not be due to different renal clearance rates (Langenbeck et al., 1980). Rather these differences may reflect various residual activities of L-phenylalanine hydroxylase or different constants of Michaelis-Menten (Km) for the phenylalanine transamination reaction (Blau et al., 1983). The differences should not be due to the efficacy of alternative elimination routes, for example to phenylacetylglutamine or to 2-hydroxyphenylacetic acid, because of the very high constants of Michaelis-Menten (Km) for these routes. Phenylalanine transamination is fully induced only after a longer-lasting load with excessive amounts of phenylalanine (Langenbeck et al., 1988). Thus (1) the transamination product phenyllactic acid could be of similar value for control of phenylketonuria as HbA1 in diabetes mellitus, although one could feel the analogy being somewhat inappropriate because HbA1 is an irreversible end-product while phenyllactic acid may be reversed to phenylpyruvic acid in very small amounts. As phenylalanine metabolites probably are a better indicator of acute neurotoxicity than concentrations of phenylalanine, and also because HPLC of phenyllactic acid is easier (and cheaper) than amino acid chromatography. (2) Plasma phenyllactic acid concentrations should be followed up closely in phenylketonuric patients: they may prove to be useful for the management of patients (as well as for studying pathogenetic mechanisms). Moreover (3): has the diet to be continued rigorously in patients with high plasma phenyllactic acid concentrations and to be reinstituted in patients with elevation of phenyllactic acid following diet termination, and may the diet be terminated in patients with persistent low phenyllactic acid?