We have carried out single-voxel proton magnetic resonance spectroscopy centered on the putamen both ipsilateral and contralateral to the worst affected side in nine subjects with drug naive idiopathic Parkinson's disease (IPD); seven chronically levodopa-treated dyskinetic IPD subjects; and 11 age-matched healthy controls. Measurements of N-acetylaspartate (NAA)/choline (Cho), NAA/(Creatine + Phosphocreatine) (Cr + PCr), and Cho/(Cr + PCr) were made. We found a significant reduction in NAA/Cho ratios from the putamen contralateral to the most affected side in the drugnaive group (p = 0.009), but not the levodopa-treated IPD groups compared with controls. There were no significant differences in NAA/(Cr + PCr) or Cho/(Cr + PCr) ratios. In untreated IPD, reduced putaminal NAA/Cho ratios may reflect loss of nigrostriatal dopamine terminals or alternatively indicate a functional abnormality of striatal putaminal neurons, such as membrane dysfunction due to striatal deafferentation. This study suggests that NAA/Cho ratios may be affected by L-dopa therapy and this may provide a reversible marker of neuronal dysfunction in the striatum.