Much evidence suggests that oxidative stress has a key role in Alzheimer's disease due to the excess of free radical production and the failure in the antioxidant mechanisms that avoid deleterious changes in cellular functions. Here we analyze circulating oxidative stress parameters and enzymatic and nonenzymatic antioxidant defense systems in patients with early stage Alzheimer's disease, according to their apolipoprotein E and angiotensin-converting enzyme insertion/deletion genotypes. We find that oxidative damage and antioxidant status depend on the genetic characteristics of the individuals, being that those with the apolipoprotein E ε4 allele but not with the angiotensin-converting enzyme insertion/deletion genotype showed the highest oxidative damage and the lowest antioxidant defenses. These results suggest the potential importance of an antioxidant supplementation not only in early stage Alzheimer's disease patients, but also for the general population as prevention, and mainly in those people with higher susceptibility to oxidative stress.