Four cases of early infantile polyneuropathy with defective myelination are reported. The peripheral nerve was studied by light and electron microscopy; different morphological characteristics have been noticed in these patients. Case 1 presented aspects of defective myelination with atypical "onion bulb" formation composed of multiple layers of basement membrane. In case 2, defective myelination and atypical "onion bulb" formation were associated with aberrant hypermyelination. Cases 3 and 4 were brothers, who presented axonal damage and atypical "onion bulb" formation.