The parkin protein functions as a RING‐type ubiquitin protein ligase. Considering the possibility that impaired ubiquitin‐proteosomal system activity may impair antioxidant defenses and enhance oxidative stress, we have investigated the activity of mitochondrial respiratory enzymes in patients with parkin gene mutations. A significant decrease in the leukocyte complex I activity was found both in patients with parkin mutations (62.5%) and idiopathic PD (64.5%) compared with age‐matched controls (P < 0.001). Complex IV activity was also decreased significantly in idiopathic PD patients (60%), but no difference was detected between controls and patients with parkin mutations. © 2003 Movement Disorder Society