An autopsy case of homocystinuria due to 5,10-methylenetetrahydrofolate reductase deficiency is presented. A 15-year-old boy had mental retardation, epilepsy, and peripheral neuropathy. A sural nerve biopsy revealed a decreased number of myelinated fibers, abnormally thick myelinated fiber groups, and numerous thinner unmyelinated fibers. The autopsy study revealed unusual findings of peripheral neuropathy and spheroid formation in addition to arterial structural abnormalities and perivascular demyelination which are common in cases of homocystinuria. The peripheral neuropathy and spheroid formation may be related to the low level of serum folic acid. The presence of peripheral neuropathy should be ascertained in cases of 5,10-methylenetetrahydrofolate reductase deficiency.