Apolipoprotein E (apoE), a protein produced by glial cells, is responsible for maintenance of the structural integrity of the microtubules within the axon of the neuron. The gene associated with apolipoprotein E, APOE, influences the construction and regeneration of the microtubules in an APOE allele-specific manner: APOE 2/2 may be neuroprotective, whereas APOE 4/4 may be neurodestructive. Thus, APOE appears to be one genetic factor that modifies the brain's response to insult, and therefore may modify the severity of neuropsychologic deficits. This article presents an overview of the genetic relation between APOE and neuropsychological function in Alzheimer disease and proposes a relation between APOE and recovery after head injury.