Objective: To determine the relative contributions of traumatic brain injury (TBI) and APOE ε4 allele status to cognitive functioning over time in people 55 years and older. Design: Observational. Setting: Research center at an urban medical center. Participants: 54 subjects with TBI and 41 participants with no history of TBI. Interventions: Not applicable. Main Outcome Measures: Factor scores measuring memory, processing speed, verbal fluency, and executive functioning derived from standardized neuropsychologic tests. Results: The neuropsychologic battery was administered twice postinjury, with a mean interval of 31.9±12 months between assessments. An initial analysis of covariance (ANCOVA), controlling for age and education, comparing the positive and negative APOE ε4 status groups yielded no significant differences between groups at either time point for any of the factors. However, when the TBI and non-TBI groups were compared, a main effect for group was found on the processing speed factor, with the TBI group performing worse than the non-TBI group at both time points (P<.001). ANCOVAs comparing both the positive and negative APOE ε4 status among TBI and non-TBI groups showed no significant between-group differences in cognitive functioning over time. Conclusions: In this study, older adults with TBI did not exhibit different rates of cognitive change over time from non-TBI counterparts and APOE ε4 status did not contribute significantly to changes in cognitive function. However, older adults with TBI did have greater cognitive impairment than their non-TBI counterparts at both assessment points on a factor measuring processing speed.