Previous reports on the activities of essential endogenous antioxidants such as superoxide dismutase, catalase, and glutathione in dementia patients have not included a simultaneous quantitative assessment of dietary antioxidant intake. This is important because the reported differences in endogenous antioxidant levels among dementia patients may have reflected variations in the total antioxidants' intake. In this study we measured the levels of antioxidant vitamins A, C, and E in the diet of 81 dementia patients and controls at the same time as assessing blood levels of three endogenous antioxidants. Results showed a significant decrease in the intake of vitamins C (p < .001) and E (p < .01) in patients with severe Alzheimer's disease (AD) when compared to controls. Patients with mild/moderate AD differed from controls only in the intake of vitamin C (p < .01). The blood levels of catalase but not superoxide dismutase and glutathione were significantly decreased in the patients with severe AD when compared to controls (p < .01), patients with mild/moderate AD (p < .01), and patients with dementia with Lewy bodies (p < .05). The blood catalase levels of dementia patients, as a whole, were significantly and positively associated with the intake of vitamins A (p < .05), C (p < .01), and E (p < .05). The results indicated that dietary intake of vitamins A, C, and E may influence blood levels of catalase possibly through their antioxidant effects on free radicals. The data underscore the importance of concurrent quantitative assessment of nutritional intake when measuring endogenous antioxidant activities and support a role for antioxidant supplementation in the treatment of dementia disorders.