The activity of the enzyme copper-zinc superoxide dismutase (Cu-Zn SOD) has been investigated in serum and red blood cells (RBC) homogenate obtained from demented patients with associated vascular lesions (VD), demented patients with probable Alzheimer's disease (DAT) and healthy controls (CG) of the same age. The increase in SOD activity was statistically significant (P < 0.01) in RBCs homogenate of DAT and VD patients, when compared to controls, but no differences appear between the two diseases groups. Additionally, a statistically significant increase in SOD activity (P < 0.01) in DAT patients above 70 years as compared to those 50-70 years old, and a relation between SOD and age were found. No changes in SOD activity with age in healthy controls nor in vascular dementia group were detected. A statistically significant increase in Circulating SOD activity (P < 0.01) was observed in vascular patients compared to controls. The observed increase in DAT Circulating SOD activity (against CG) was not significant. The increased levels of Cu-Zn SOD, probably represent a general alteration of the oxidative processes characteristic of these dementias and suggest that the enzyme might be used as a marker.