Objective: To examine changes in motivation and the relationship of motivation to adherence to self-monitoring and weight loss in a 16-week Internet behavioral weight-loss intervention. Design: Two-group randomized design. Setting: This study was conducted over the Internet. Participants: Sixty-six women, ages 22-65, with a body mass index (BMI) between 25 and 40, and with a home computer with Internet access, participated. Intervention: These data are from an Internet behavioral weight-loss program in which all participants received a face-to-face initial session followed by the 16-week Internet program. Main Outcome Measures: Adherence to self-monitoring and weight loss were the main outcome measures. Analysis: Linear regression and mediation analysis using the Baron and Kenny method were conducted. Results: Autonomous motivation increased initially and remained high for those who ultimately achieved a 5% weight loss, but declined over time for participants who did not achieve a 5% weight loss. Autonomous motivation at 4 weeks was a predictor of adherence to self-monitoring and 16-week weight loss. Adherence mediated the relationship between autonomous motivation and weight loss. Conclusions and Implications: These results suggest that autonomous motivation may be a promising intervention target for promoting program procedure adherence and weight loss. (Contains 2 figures and 1 table.)