There is growing evidence for the involvement of oxidative stress and mitochondrial respiratory failure in nigral neuronal death in Parkinson's disease (PD). We report increased immunoreactivity of 8‐oxo‐dGTPase (8‐oxo‐7, 8‐dihydrodeoxyguanosine triphosphatase [hMTH1]), an enzyme known to play an important role in controlling spontaneous mutagenesis, and 8‐oxo‐7, 8‐deoxyguanosine (8‐oxo‐dG) in the mitochondria of the substantia nigra of 6 PD patients. Our results suggest that hMTH1 might be a useful marker of oxidative stress and can be used to explore the relation between such oxidative stress and genomic instability.