Giant axonal neuropathy is a progressive central-peripheral axonopathy characterized by distention of axons by aggregated neurofilaments. We report two female siblings with giant axonal neuropathy. Both patients developed symptoms of a chronic progressive polyneuropathy at age 3 years. Clinical evidence of central nervous system involvement was present in both cases. Autopsy neuropathologic examination of the older sibling at the age of 11 years revealed numerous giant axons, Rosenthal fibers, and gliosis throughout the brain and spinal cord and typical giant axons in the peripheral nerves. Electrophysiologic studies in the younger sibling indicated brain stem dysfunction, and her sural nerve biopsy revealed enlarged axons packed with neurofilaments. These patients illustrate that neurologic deficits of giant axonal neuropathy result from widespread lesions in the central, as well as peripheral (including autonomic), nervous systems. This occurrence of giant axonal neuropathy in two siblings supports a genetic origin of this disease. This is the first report of autopsy findings in giant axonal neuropathy in an affected sibling. (J Child Neurol 1990;5:229-234).