This study tested the hypothesis that flight-phobic patients experience change at different rates even when they are receiving identical treatment. Faster within-session rates of change (WSRC) were expected for patients who required fewer exposure sessions. The study also tested the theoretical role of autonomic flexibility on WSRC. High flexibility should be associated with faster rates of change. Thirty-seven flight-phobic patients were successfully treated with a computer-assisted fear of flying treatment. A significant negative correlation was found between total number of sessions and WSRC: The fewer sessions patients attended, the faster their rate of change was. The role of autonomic flexibility was partially supported: A significant correlation between heart rate variability and WSRC revealed that flexible patients improved faster than less-flexible patients. © 2011 Wiley Periodicals, Inc. J Clin Psychol 67:1–11, 2011.