The translation of promising preclinical treatments into effective drugs for Alzheimer's disease (AD) has been challenging. One of the most potent risk factors for sporadic AD is carrier status of the epsilon 4 allele of the apolipoprotein E gene (E4). E4 carriers show a differential response to several therapies which are being investigated as AD treatments, including acetylcholinesterase inhibitors and therapeutics with vascular and metabolic targets. The differential treatment responses of E4 carriers may partially explain why some treatments show a null effect in clinical trials. Understanding the reasons behind these responses is not only important for clinical practice, but may also help us elucidate mechanisms for this neurodegenerative disease.