We report the clinical and post-mortem findings in a 4-day-old female infant with methylmalonic acidemia (MMA) prematurely delivered at 36 weeks gestation. Complementation analysis revealed a defect in methylmalonyl-CoA mutase apoenzyme activity. There was selective cell death of immature neurons involving germinal matrix, migrating neuroblasts, and both external and internal granular cell layers of the cerebellum and dentate gyrus of the hippocampus. Karyorrhexis was also seen in the lymphoid follicles of spleen, lymph nodes and in the thymus. Additionally, the spinal nerve roots demonstrated optically empty spaces in their myelin sheaths due to adaxonal separation of their myelin lamellae.