In classic galactosemia, long-term neurologic sequelae can include low cognitive functioning and a curious neurologic syndrome with tremors, dysmetria, and ataxia. An abnormal white-matter signal on cerebral magnetic resonance imaging (MRI) is present in almost all patients; some have mild cerebral or cerebellar atrophy and focal white-matter lesions. The present study was undertaken to assess the integrity of myelinated pathways by recording somatosensory evoked potentials. Results were correlated with age at diagnosis, severity of illness, age at evoked potentials, neurologic examination, MRI studies and cognitive outcome as measured by the Woodcock-Johnson Revised Standard Cognitive Battery. Evoked potentials were abnormal in 17 (28%) of 60 patients who had median nerve, and 26 (77%) of 34 patients who had posterior tibial nerve studies. Abnormalities of the central rather than the peripheral nervous system were most common. Evoked potentials correlated with severity of presenting symptoms (P = .011), age at evoked potential testing (P = .029), and presence of focal white-matter lesions on MRI (P = .049). Results of neurophysiologic testing showed no correlation with the Woodcock-Johnson Battery. Patients with classic galactosemia may have abnormal conduction along myelinated pathways that is associated with other central deficits. Myelin, which contains galactose, may be adversely affected in this inborn error of metabolism.