Fatigue is one of the most frequent symptoms in cancer patients. However, the precise causes of this fatigue are still unknown, and this situation makes it difficult to combat the problem. The present study was conducted to investigate factors correlated with fatigue in disease-free breast cancer patients. A group of 134 randomly selected ambulatory breast cancer patients who had undergone successful surgical treatment participated. They completed the Cancer Fatigue Scale, the Hospital Anxiety and Depression Scale, the Mental Adjustment to Cancer Scale, and an ad hoc questionnaire detailing physical symptoms, social support, and demographic variables at home and returned them by mail the following day. Multiple regression analysis revealed that fatigue was significantly correlated with dyspnea, insufficient sleep, and depression, and that these three variables accounted for a total of 46% of variance in fatigue. Factors concerned with the cancer and treatment, such as disease stage, lymph node metastasis, number of days since operation, past intravenous chemotherapy, radiotherapy, current use of fluoropyrimidine compounds, and current use of tamoxifen citrate were not correlated with fatigue. The results suggest that fatigue in this population is determined by current physical and psychological distress rather than by the cancer itself and prior cancer treatments, and that the management of dyspnea, insomnia, and depression might be important in reducing fatigue in this population.