We present an autopsy case of ornithine transcarbamylase (OTC) deficiency with grumose degeneration in the dentate nucleus of the cerebellum. The patient had intractable neonatal convulsions and hyperammonemia from the 3rd day after birth. Diagnosis of OTC deficiency was made based on null activity of the enzyme and four-base deletions in exon 9 of the OTC gene. Death was due to sepsis as well as disseminated intravascular coagulation at 1 year and 2 months of age. Neuropathology showed multiple cystic changes and ulegyria in the bilateral frontal and parietal lobes. Multiple cysts were associated with the region, which was infiltrated with macrophages surrounded by astroglia showing palisading pattern. Ferrugination was marked in the thalamus and severe neuronal loss with astrogliotic change in the CA1-2 area of the hippocampus. Grumose degeneration was noted in the dentate nucleus of the cerebellum. This is the first report of grumose degeneration in OTC deficiency.