Objective and Participants: The authors assessed the stability of diet and physical activity and their relationship to weight changes in first-year university women. Methods: They collected anthropometric and body composition data from 101 resident women at the beginning of their first year of college and again at 12 months. The authors obtained physical activity and dietary logs 4 times throughout the year. Results: Caloric intake decreased over 12 months in all participants (p = 0.01). There was little change in physical activity in participants who lost weight (p = 0.73, d = 0.18). Those who gained weight experienced a trend toward decreased physical activity (p = 0.13, d = 0.38). A significant Time X Group interaction on physical activity (p = 0.04) suggests that physical activity patterns differed substantially between individuals who gained weight and those who lost weight. Conclusions: Reduction in physical activity appears to be the defining characteristic in freshman weight gain. (Contains 1 table and 2 figures.)