This study describes a self-efficacy (SE) scale and examines its usefulness in predicting adherence to cognitive-behavioral therapy (CBT) of insomnia. The SE scale, which included three subscales (global, task-related, and self-regulation), was administered to 39 patients receiving CBT for primary insomnia. Participants'daily sleep diaries were used to estimate their adherence to treatment. The results showed that the SE scale has adequate psychometric properties. Treatment adherence increased from Week 2 to Week 7 and decreased during the week following the last therapy session. In general, the global self-efficacy measure was more strongly associated with adherence during the first week of treatment, whereas the task-related efficacy was more strongly associated with adherence behaviors from Weeks 4 to 8. These findings suggest that self-efficacy perceptions are useful in predicting adherence to CBT of insomnia.