A case of connatal polyneuropathy is described in a boy who died of pneumonia at the age of 2 years, and from whom sural nerve biopsies had been taken when he was 4 and 16 months old. Clinically, his disease was characterized by motor weakness and muscular flaccidity in the presence of normal intellectual development. The evolution of the connatal peripheral nerve lesion could be followed from the age of 4 months to death: The first biopsy evidenced the most serious pathologic changes. The findings were reminiscent of those encountered in a fetal nerve at 18 weeks of gestation. Furthermore, it showed numerous filamentous inclusions in Schwann cells. The second biopsy showed a sparsely myelinated nerve with bands of basement membrane apparently unrelated to cells arranged around the nerve's fibers. A few Schwann cells containing filamentous inclusions were still present. At autopsy, the findings were identical to those of the second biopsy. The possibility that this patient was transitionally exposed to a neurotoxic agent during pregnancy and that the biopsy findings represent a lesion that is still florid in the first and in a residual state in the second biopsy is considered.