The occurrence of aberrant peripheral nerve (PN) fibers in otherwise normal medullas was studied in 1,016 consecutively autopsied patients. Fine, myelinated PN fibers occurred as small parallel bundles or microneuromas about arteries and arterioles exclusively in the dorsal half of the medulla. They were more common in male than female patients, but were not related to any specific systemic disease process. The PN lesions were not found in patients under 20 years of age, but were observed in 1.5, 9.6, and 16.4% of those patients 20-49, 50-69, and 70-89 years of age, respectively. We regard these lesions as aberrant nerve fibers regenerated from cranial nerve roots rather than normally developed vascular nerves. We postulate that cranial nerve roots are subject to shearing or compression by strenuous motion of the neck in the course of ordinary activities and that the nerve fibers thus severed regenerate and grow into the pia-arachnoid and perivascular spaces to form the PN lesions. These PN lesions may become exaggerated under various abnormal circumstances such as overt traumatic injury and long-standing diseases affecting the medulla or cranial nerve roots.