We studied visual (VEP) and brainstem auditory (BAEP) evoked potential changes in 23 patients with early onset Parkinson's disease (EOPD) to establish the nature of the changes as well as their relationship to dopaminergic (DA) and serotonergic (5-HT) disturbances, as determined by cerebrospinal fluid levels of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA). We also compared these parameters between the young onset (YOPD) and juvenile Parkinsonism (JP), the two subgroups of EOPD, to look for any possible differences between the two. In EOPD, the mean P100 latency of the VEP was significantly prolonged compared to controls (p < 0.001). However, within EOPD the evoked potential parameters were not significantly different between YOPD and the JP subgroups. P100 latency was abnormal in six patients (YOPD: 5, JP: 1) (26%). Six patients (YOPD: 3, JP:3) (26%) had abnormal BAEP. A significant negative correlation (r: -0.89, p < 1%) was observed between the P100 latency and CSF HVA levels. No correlation was observed between the BAEP interpeak latencies and either CSF HVA or 5-HIAA levels. This study suggests that VEP and BAEP abnormalities do occur in EOPD (in both YOPD and JP), and that the prolongation of P100 latency is secondary to DA deficiency as in PD. The cause of BAEP abnormalities is probably independent of DA and 5-HT disturbances. The only difference between EOPD and classical PD was the higher incidence of BAEP abnormalities in EOPD. There was no correlation between the VEP or BAEP changes to either the age at onset or duration of EOPD.