Between the year 2000 and the year 2030, the percentage of U.S. adults aged 65 years and older is expected to increase from 13% of the total population to 20% of the total population. Mobility (i.e., the ability to move freely) affects quality of life and mobility limitations are associated with increased age. To maintain mobility, regular and sustained physical activity is deemed an important health behavior. In some studies, worry about health has been found to be a motivator for engaging in certain health-related behaviors, whereas in other studies it has been found to be a barrier. The relationship between older adults' physical activity participation and level of worry has received minimal research attention. This study's first purpose was to examine the mediating effect of exercise behavior on the relation between health worry and the likelihood of having walking difficulty using a nationally representative sample of older adults (M age=76.83) from the Longitudinal Study of Aging (N=7,527). This study's second purpose was to explore whether worry about health predicted the rate of change in the likelihood of having walking difficulty across time. A multiple indicator linear growth model was used. Multiple imputations and weighted least square estimation were used for estimating missing values. Walking difficulty was measured with three dichotomous items at four time points, two years apart. The mediation model was tested on the initial likelihood of having walking difficulty. Demographics, medical history, history of falls, perceived health, perceived health control, and health worry were used to predict the rate of change of the likelihood. The model fit for the mediation model was satisfactory (mean CFI=.97). People with higher levels of worry are less likely to engage in physical activity and hence, are more vulnerable for having walking difficulty. In addition, health worry predicted a faster progression on the likelihood of having walking difficulty, however, the effect was not substantially significant (β=.06, p<.001). Results suggest that health worry has both direct and indirect effects on the likelihood of having walking difficulty. Besides exercise, other health-related behaviors may also mediate the process. The results further suggest that the media, family members, physicians, and fitness instructors should not try to encourage physical activity by inducing threat and worry among older adults. Instead, assisting older adults manage and cope with their worries about health may help to increase their participation in physical activity.Keyword(s): older adult/aging issues, physical activity, wellness/disease prevention