Mild cognitive impairment (MCI) is a nosological entity proposed as an intermediate state between normal aging and dementia. MCI seems to represent an early stage of Alzheimer's disease (AD) and there is a great interest in the relationship between MCI and the progression to AD. Some studies have demonstrated an accumulation of products of free radical damage in the central nervous system and in the peripheral tissues of subjects with AD or mild cognitive impairment. The aim of the present work was to evaluate the serum levels of some enzymatic antioxidant defences like superoxide dismutase (SOD) and glutathione peroxidase (GPX), as well as lipid peroxidation markers like MDA (malondialdehyde), in MCI and AD patients, compared with age-matched healthy controls. The subjects of this study (45 patients) consisted of 15 individuals with mild cognitive impairment (MCI), 15 with Alzheimer's disease (AD) and 15 healthy age-matched controls. Biochemical analyses showed a similar decrease of the main enzymatic antioxidant defences (SOD and GPX) and increased production of lipid peroxidation marker (MDA) in the serum of the MCI and AD patients, compared to age-matched control group. This study clearly demonstrates that oxidative stress damage occurs in patients with MCI and AD. Moreover, some enzymatic markers of oxidative stress are similar in MCI and AD patients, suggesting that oxidative damage could be one important aspect for the onset of AD.