Aldehyde dehydrogenases (ALDH) play a key role in neuronal protection. They exert this function by metabolizing biogenic amine-related aldehydes, e.g. 3,4-dihydroxyphenylacetaldehyde (DOPAL), and by protecting neurons against aldehyde- and oxidative stress-related neurotoxicity. The role of these different isoenzymes has been discussed in other neurodegenerative disorders before. It is somewhat surprising that only few studies have investigated their role in the aetiology of Parkinson's disease (PD), in both the degeneration of dopaminergic neurons and the formation of Lewy bodies. Earlier studies report severe alterations of the cytosolic isoform of ALDH expression (ALDH 1A1) in the substantia nigra of patients with PD. However, there are no data regarding the activity of ALDH 2 located at the inner mitochondrial membrane. Since mitochondrial dysfunctions are hypothesized to be of importance in the aetiology of PD we have examined the enzymatic activity of mitochondrial ALDH 2 in post-mortem putamen and frontal cortex of patients with PD and controls. We found that mitochondrial ALDH 2 activity in contrast to the frontal cortex was significantly increased in the putamen of patients with PD compared to controls.