BACKGROUND: Adherence to protocols of accelerometer use by participants of research studies is crucial to ensure the most accurate measure of their physical activity. METHODS: We used data from a study of 201 individuals 65 years of age and older to examine whether aging effects on physical and cognitive health limit the ability of an older adult to be adherent to an accelerometer protocol. RESULTS: A comparison of participants who met the adherent person criteria with those who did not showed that the percentage of participants whose income is $20,000 or greater, the percentage of participants who reported white race, and the mean number of school grades completed were significantly different between the 2 groups. Logistic regression analyses showed that the best multivariate model to predict being a valid person included Instrumental Activities of Daily Living score, while the best multivariate model to predict being an adherent person included Modified Guralnik Lower Body Score and Mini-Mental State Examination Score. CONCLUSIONS: This study found that certain measures of physical and cognitive functioning were the best predictors of adherence to an accelerometer protocol among older adults.