STUDY OBJECTIVES: This study was conducted to exam the degree to which cognitive-behavioral insomnia therapy (CBT) reduces dysfunctional beliefs about sleep and to determine if such cognitive changes correlate with sleep improvements. DESIGN: The study used a double-blind, placebo-controlled design in which participants were randomized to CBT, progressive muscle relaxation training or a sham behavioral intervention. Each treatment was provided in 6 weekly, 30-60-minute individual therapy sessions. SETTING: The sleep disorders center of a large university medical center. PARTICIPANTS: Seventy-five individuals (ages 40 to 80 years of age) who met strict criteria for persistent primary sleep-maintenance insomnia were enrolled in this trial. INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: Participants completed the Dysfunctional Beliefs and Attitudes About Sleep (DBAS) Scale, as well as other assessment procedures before treatment, shortly after treatment, and at a six-month follow-up. Items composing a factor-analytically derived DBAS short form (DBAS-SF) were then used to compare treatment groups across time points. Results showed CBT produced larger changes on the DBAS-SF than did the other treatments, and these changes endured through the follow-up period. Moreover, these cognitive changes were correlated with improvements noted on both objective and subjective measures of insomnia symptoms, particularly within the CBT group. CONCLUSIONS: CBT is effective for reducing dysfunctional beliefs about sleep and such changes are associated with other positive outcomes in insomnia treatment.