Lower consumption of docosahexaenoic acid (DHA) is commonly but not always associated with higher risk of cognitive decline and diagnosis of Alzheimer's disease (AD). We review here the available data relating DHA to AD, with emphasis on DHA content of plasma and brain. Our assessment of this literature is that low DHA is not consistently observed in AD plasma or brain. However, in dietary and population studies, low DHA intake is usually associated with low plasma DHA. Therefore, at present, there is no clear explanation of why the usual low DHA intake-low plasma DHA relationship appears not to exist in AD. Adding to the confusion, preliminary and inconclusive reports tentatively suggest that dietary DHA could potentially reduce cognitive deterioration in AD. These inconsistencies between dietary DHA, plasma/tissue DHA, and possible DHA efficacy in AD may be more methodological than biological, and may arise in part because only one study to date has reported both DHA intake and plasma DHA values in the same AD patients. Studies reporting DHA intake and plasma levels while also undertaking a DHA intervention in AD would presumably help resolve these issues.