We report a 66-year-old woman who developed mental deterioration in her school days, and progressive gait disturbance, dysarthria and bradykinesia in her 40 s. Her parents were consanguineous, and the two of her brothers were suspected to have the allied disorder. On physical examination, she was short-statured and high-arched palate was observed. Neurological examination revealed dementia, abnormal eye movement, dysarthria, spastic paraparesis with hyperreflexia, bilateral Babinski signs, cerebellar ataxia and dysuria. Brain MRI showed marked hypoplasia of corpus callosum with dilatation of lateral ventricles and cerebral sulci and significant cerebellar atrophy. Amino acid analyses showed significant elevation of glycine without ketosis in serum, cerebrospinal fluid, and urine, which lead us to the diagnosis of late-onset nonketotic hyperglycinemia(NKH). NKH is known to be a rare autosomal recessive metabolic disorder primarily caused by deficient activity of various components of the mitochondrial glycine cleavage system. Onset of the disease occurs most often in early infancy, however, later-onset variants have been described. Usually, late-onset NKH only manifests mild mental deterioration, character change, seizure, ataxia or spastic paraparesis, which sometimes makes difficulty in differentiating this disease from other hereditary cerebellar ataxia or spastic paraparesis. In addition, many structural brain abnormalities have been reported accompanied with NKH, and especially, agenesis or hypoplasia of corpus callosum is the most characteristic feature in this disease. Therefore, we emphasized that amino acid analyses should be considered in any patients who have cerebellar ataxia or spastic paraparesis of unknown cause with these neuroradiological findings.