This study evaluated adherence to group cognitive behavioral treatment in 50 adults with chronic insomnia. Adherence was measured using questionnaire data, consistency of sleep scheduling, and % of sessions attended. Results showed that therapists' rated 48% of participants as "very much" to "extremely" adherent. Using stepwise regression, only therapist-rated adherence explained a significant amount of variance in post-treatment outcome. Therapist-rated adherence predicted post-treatment ratings of sleep-related impairment, dysfunctional beliefs about sleep, and overall sleep quality (but not actual sleep duration or efficiency). Using a multivariate analysis of variance (MANOVA) procedure, results revealed that a diagnosis of dysthymia, based on a structured clinical interview, was associated with reduced adherence and less improvement in sleep-onset latency and sleep efficiency, but that scores on a dimensional measure of depression were not associated with either adherence or outcome. Implications of these findings are that the practice of treatment techniques is related to an improved perception of sleep and more healthy and appropriate beliefs about the causes of poor sleep. Therapists should continue to pay close attention to the adherence behavior of those with insomnia, particularly if they are depressed.