BACKGROUND: Breast cancer is the second most common cancer in Korean women. As survival years increase, health-related quality of life has become an important issue in breast cancer patients. Sleep problems are common and cause significant disruption in quality of life in breast cancer patients. However, cancer-related sleep disturbance has received little attention. The purpose of this study was to determine the prevalence of poor sleep quality and factors which are associated with poor sleep quality in the breast cancer patients receiving chemotherapy in the outpatients setting. METHODS: One hundred and twenty-seven breast cancer patients receiving chemotherapy in a tertiary hospital outpatient were surveyed between February 2009 and July 2009. Among them, 94 (72.8%) patients were finally included in the study. The sleep quality was assessed by the Pittsburg Sleep Quality Index (PSQI). PSQI > 5 indicates clinically significant poor sleep quality. Also the independent factors of sleep quality were assessed using univariable analysis and multiple logistic regression analysis. RESULTS: Seventy-two (76.6%) patients of 94 breast cancer patients undergoing chemotherapy have poor sleep quality. Among of them, 11 (15.3%) patients were actually consulted with doctors. Average PSQI score was 8.8 (+/- 4.1). Anxiety and employment status were associated with poor sleep quality. CONCLUSION: A high proportion of breast cancer patients receiving chemotherapy even in the outpatient settings had poor sleep quality. But only small proportion of them consulted doctor. Poor sleep quality during chemotherapy in breast cancer patient was associated with anxiety and employment status. Considering the high prevalence of sleep problem and inadequate management, more adequate attention is needed to manage the sleep problem of breast cancer patients receiving chemotherapy.