It is well known that coenzyme Q (CoQ) has important antioxidant properties. Because one of the main mechanisms involved in the pathogenesis of Alzheimer's disease (AD) and other neurodegenerative diseases is oxidative stress, analysis of the concentrations of CoQ in different tissues of AD patients and with other dementia syndromes and the possible therapeutic role of CoQ in AD have been addressed in several studies. We performed a systematic review and a meta-analysis of these studies measuring tissue CoQ levels in patients with dementia and controls which showed that, compared with controls, AD patients had similar serum/plasma CoQ levels. We also revised the possible therapeutic effects of CoQ in experimental models of AD and other dementias (which showed important neuroprotective effects of coenzyme Q) and in humans with AD, other dementias, and mild cognitive impairment (with inconclusive results). The potential role of CoQ treatment in AD and in improving memory in aged rodents shown in experimental models deserves future studies in patients with AD, other causes of dementia, and mild cognitive impairment.