Dentato-rubro-pallido-luysian atrophy (DRP-LA) is a rare autosomal dominant neurodegenerative disorder. Recently the genetic abnormality has become clear. We encountered four patients with DRPLA (including a 14-year-old boy and his father) in whom the final diagnosis was made by DNA analysis. In addition to performing conventional MRI, we quantified brain metabolites by proton MR spectroscopy. We also compared the expanded repeat size of CAG trinucleotide in a gene on the short arm of chromosome 12 to the MR findings which consisted of the findings of clinical severity and age of onset. The method reported by Nagafuchi et al. was used to determine the size of the CAG repeat on the focus chromosome. Repeat size was closely correlated with age at onset and disease severity. The MR studies were performed with a Magnetum H-15SP (Siemens, 1.5 Tesla) equipped with a C-P type head coil. Axial T2-WIs (TR = 2000 ms, TE = 90 ms) and T1-WIs (TR = 200 ms, TE = 15 ms) were measured. Atrophy of the brainstem, the tegmentum, and the cerebellum, and periventricular hyperintensity could be seen. The sequence for 1H-MRS was PRESS, voxel size was 8 ml, and regions of interest were placed on right basal ganglia and right parietal white matter. After compensating the observed signals, we obtained metabolite concentrations by using compensated water intensity as an internal standard. Despite the absence of abnormalities of the basal ganglia on MRI, NAA concentrations differed from patient to patient and were low in severe clinical cases. We propose that the CAG repeat size of the focus chromosome and NAA concentration quantified by 1H-MRS are closely correlated with clinical severity and age of onset. In conclusion, DNA analysis may be useful in early diagnosis and 1H-MRS can detect metabolic abnormalities non-invasively even when no markedly abnormal findings can be detected with other clinical modalities.