Mutations of the proteolipid protein (Plp) gene cause a generalized central nervous system (CNS) myelin deficit in Pelizaeus‐Merzbacher disease of man and various tremor syndromes in animal models. X‐linked spastic paraplegia is also due to Plp gene mutations but has a different clinical profile and more restricted pathology involving specific tracts and regions. We have shown previously that PLP overexpression in mice homozygous for a Plp transgene results in premature arrest of CNS myelination and premature death. Here, we demonstrate that a low‐level increase in Plp gene expression in transgenic mice causes significant axonal degeneration and demyelination with predilection for specific tracts. Following normal motor development, aged mice develop progressive myelin loss, axonal swellings with resultant Wallerian degeneration, and marked vacuolation of the neuropil associated with ataxia, tremor, and seizures. The age of onset and severity of the phenotype is a function of Plp gene dosage. The corticospinal tracts, optic nerve, fasciculus gracilis cerebellum, and brainstem are particularly involved. Although oligodendrocyte cell bodies show little abnormality, their inner adaxonal tongue is often abnormal, suggesting a perturbation of the axon/glial interface that may underlie the axonal changes. We conclude that abnormal expression of an oligodendrocyte‐specific gene can cause axonal damage, a finding that is relevant to the pathogenesis of PLP‐associated disorders and probably to other myelin‐related diseases. J. Comp. Neurol. 394:506–519, 1998. © 1998 Wiley‐Liss, Inc.