This study examined the relationship of catastrophizing to fatigue in 80 women receiving chemotherapy (CT) or radiotherapy (RT) for treatment of early stage breast cancer. Findings revealed expected relationships between catastrophizing and fatigue among women receiving RT but not CT. Among RT patients, those high in catastrophizing reported significantly greater fatigue than was reported by those low in catastrophizing; among CT patients, differences in fatigue based on level of catastrophizing were not significant. Likewise, catastrophizing was found to account for significant variability in subsequent reports of fatigue among RT patients but not CT patients. These findings extend research on catastrophizing beyond previously studied relationships with pain and are consistent with the view that, as the inherent symptom-producing potential of treatment decreases, psychological factors play a greater role in patients' experience of symptoms.