Mixed types of sensory and motor peripheral neuropathy and dementia occurred as neurological complications in patients with established type IV and type V hyperlipoproteinemia. These complications were remedial by control of the hyperlipoproteinemia with diet and/or clofibrate resulting in symptomatic improvement as well as restitution of nerve conduction velocities toward normal. Diabetes mellitus as well as systemic metabolic and toxic disorders which commonly produce neurological complications were excluded. Segmental demyelination with disorganization of myelin lamellae were striking morphological features found on sural nerve biopsy. Fluctuations in memory performance correlated inversely with plasma lipid levels and appear to be a characteristic feature of hyperlipidemic dementia. Hyperlipidemic neuropathy and dementia, although rare, are remediable neurological disorders which should be considered in patients with neuropathy and/or dementia of unknown origin.