We studied the pattern-reversal VEP, the BAEP and the EEG with conventional and computerized analysis in 41 adolescents with hyperphenylalaninemia (24 HPA type 1, 12 HPA type 2, 4 HPA type 3, 1 DHPR deficiency) and 35 control persons. A prolongation of the P100 latency in the VEP, a greater interear difference of the I-V interwave latency in the BAEP and slowing of EEG background activity were found. Five per cent of the patients demonstrated spikes and 12.5% abnormal sharp transients in the EEG. The latency increase in the VEP corresponded to the compliance with the diet during the first decade of life. No influence of the actual Phe concentration at the time of the investigation was demonstrated. The BAEP- and EEG-findings were not related to the course of treatment. Thus the VEP changes in this cross sectional study refer to alterations of brain function that occurred during the early years of life. To investigate the value of evoked potentials and EEG in monitoring brain function after discontinuing the diet longitudinal data are needed.