Studies of therapist adherence in relation to treatment outcome have produced mixed results. The aim of the present study was to investigate change in therapist adherence to cognitive–behavioral therapy (CBT) for bulimia nervosa over time, and to investigate the relationship between adherence and client outcome in early, middle, and late phases of treatment. Thirty-six clients received the focused form of “enhanced” CBT (CBT-E) for bulimia nervosa. Trained observers rated audiotapes of 92 full-length therapy sessions from early (Session 3), middle (Session 11), and late phases (Session 20) of treatment using the Cognitive–Behavioral Therapy Treatment Protocol Adherence Scale. Change in adherence across the 3 treatment phases was examined using multilevel analysis. The relationship between early, middle, and late adherence levels and end-of-treatment binging frequency was examined using multilevel Poisson regression analysis. Adherence decreased significantly over the course of treatment. Higher levels of therapist adherence in early and middle phases of treatment were associated with reduced binging frequency, whereas higher levels of adherence measured late in treatment was not. Results indicate that therapists’ adherence to the CBT-E treatment protocol decreases over time and that high levels of protocol adherence in early and middle phases of treatment are more important for positive client outcomes than high levels of adherence in the end of treatment.