This study investigated the characteristics, course, and correlates of fatigue in women receiving adjuvant chemotherapy for breast cancer. Fifty-four patients were assessed before the start of chemotherapy and during the first three treatment cycles. An age-matched sample of women with no cancer history was assessed at similar time intervals for comparison purposes. Results indicated that breast cancer patients experienced worse fatigue than women with no cancer history. These differences were evident before and after patients started chemotherapy. In addition, fatigue worsened among patients after treatment started. More severe fatigue before treatment was associated with poorer performance status and the presence of fatigue-related symptoms (e.g., sleep problems and muscle weakness). Increases in fatigue after chemotherapy started were associated with continued fatigue-related symptoms and the development of chemotherapy side effects (e.g., nausea and mouth sores). These findings demonstrate the clinical significance of fatigue in breast cancer patients before and during adjuvant chemotherapy treatment. Results also suggest that aggressive management of common side effects, such as nausea and pain, may be useful in relieving chemotherapy-related fatigue.