The purpose was to examine psychosocial factors that influence the physical activity behaviors of adults with brain injuries. Two differing models, based on Harter's model of self-worth, were proposed to examine the relationship between perceived competence, social support, physical self-worth, affect, and motivation. Adults numbering 384 with brain injuries completed a series of questionnaires measuring each psychosocial variable. The structural analysis indicated a nonsignificant chi squared value and good fit indices for model two which included affect as the mediating variable. Findings indicate that affect is critical in shaping the physical activity cognitions and behaviors of adults with brain injuries. Suggestions are made on practical ways to enhance affect and subsequently physical activity participation. (Contains 2 tables and 2 figures.)