Apolipoprotein E (apo E) is a lipoprotein produced by astrocytes and microglia and has a proposed role in transporting lipids to injured neurons. There are three known isoforms of apo E, coded for by the APOE epsilon2, APOE epsilon3, and APOE epsilon4 genes. The APOE epsilon4 genotype has been implicated as a risk factor for Alzheimer's disease. Recent studies have suggested that APOE epsilon4 may influence the central nervous system's response to injury. This article presents an overview of the relationship between apo E, Alzheimer's disease, and head injury and reviews recent studies implicating APOE epsilon4 as a possible genetic determinant in recovery from head injury.