Behavioral dysfunction is a problem in patients with Alzheimer disease (AD), and is apparent in up to 67% of individuals. Such changes are a primary cause of individual institutionalization and often lead to their functional disability. As AD progresses, the worsening of behavioral dysfunction becomes increasingly evident and is linked with decreased patient survival. Unfortunately, some of the more common drug therapies used in AD patients to stabilize other facets of their disease worsen behavioral dysfunction. Behavioral changes are associated with endogenous and exogenous factors such as disease stage, environmental factors, other medical conditions, drug regimen, and AD genotype. The most commonly examined and important genotype in AD is the apolipoprotein E (APO E) series, and APO E genotyping is also a useful diagnostic tool. The most frequent APO E genotypes encountered in AD are APO E-4/4, APO E-3/4, and APO E-3/3. In the current study, AD behavioral dysfunction, anxiety, and psychoses were commonly associated with the APO E-3/3 genotype, whereas disorientation, agitation, depression and motor disorders were common among patients with the APO E-4/4 and APO E-3/4 genotypes. These differences were not statistically significant but they suggest that different APO E genotypes influence the phenotypic expression of specific noncognitive symptoms, including behavioral function, in AD.