PURPOSE: Fatigue is one of the most frequent symptoms in patients with cancer. However, the precise determinants of fatigue are still unknown. This study was conducted to investigate factors correlated with cancer-related fatigue before surgery and just before subsequent adjuvant therapy. METHODS: Patients completed the Multidimensional Fatigue Inventory (MFI-20), the European Organization for Research and Treatment of Cancer 30-item quality-of-life questionnaire before and after surgery, the Trait Anxiety Inventory and the Life Orientation Test before surgery, and the State Anxiety Inventory before the start of adjuvant therapy. Multiple regression analysis of determinants of change in MFI-20 total score after surgery was conducted. RESULTS: A series of 466 eligible patients with stage I-III breast cancer with planned surgery were recruited. An increase in MFI-20 total score after surgery was significantly correlated with higher preoperative fatigue and lower role functioning before surgery; a decrease in role functioning, physical functioning, and cognitive functioning after surgery; an increase in insomnia after surgery; and a higher state anxiety after surgery. Disease stage, lymph node metastases, surgical procedure, and demographic characteristics (e.g., age, marital status, having children, educational level) were not correlated with fatigue in multivariate analysis. CONCLUSION: These results suggest that worsening fatigue after surgery for breast cancer is associated with a decrease in physical functioning and an increase in psychological distress rather than with the cancer characteristics. Therefore, screening measures should be implemented at the time of diagnosis-before starting treatment-to identify psychologically vulnerable patients and to offer them professional support.