Prior studies have supported the efficacy of cognitive behavioral therapy (CBT) for insomnia comorbid with cancer. This article reports secondary analyses that were performed on one of these studies to investigate the predictive role of changes in dysfunctional beliefs about sleep, adherence to behavioral strategies, and some nonspecific factors on sleep changes assessed subjectively and objectively. Fifty-seven women with chronic insomnia comorbid with breast cancer received CBT for insomnia. At posttreatment, subjective sleep improvements were best predicted by higher initial levels of treatment expectancies, but also by decreased dysfunctional beliefs about sleep; the most consistent predictors of polysomnography (PSG) assessed sleep improvements were reduced dysfunctional beliefs about sleep and a higher avoidance of day napping. At 6-month follow-up, subjectively assessed sleep improvements were best predicted by adherence to behavioral strategies, whereas none of the predictors was significantly associated with PSG-assessed sleep improvements. This study gives some support to the importance of targeting erroneous beliefs about sleep and poor sleep habits in the treatment of cancer-related insomnia, but also to the importance of enhancing patients' expectancies for improvement. (Contains 6 tables.)