Objective: The diagnosis of breast cancer, the most common type of cancer among American women, elicits greater distress than any other diagnosis regardless of prognosis. Therefore, the present study examined the efficacy of a stress reduction intervention for women with breast cancer. Methods: As part of a larger, randomized, controlled study of the effects on measures of stress of a mindfulness-based stress reduction (MBSR) intervention for women with breast cancer, the current analyses examined the effects on sleep complaints. Results: Analyses of the data indicated that both MBSR and a free choice (FC) control condition produced significant improvement on daily diary sleep quality measures though neither showed significant improvement on sleep-efficiency. Participants in the MBSR who reported greater mindfulness practice improved significantly more on the sleep quality measure most strongly associated with distress. Conclusion: MBSR appears to be a promising intervention to improve the quality of sleep in woman with breast cancer whose sleep complaints are due to stress.