Although much has been learned about the complication of fatigue during breast cancer treatment, the possibility that there are differences across treatment modalities in breast cancer patients' experience of fatigue has not yet been established. In this study, fatigue was assessed in 134 women receiving chemotherapy and radiotherapy or radiotherapy only for early stage breast cancer. Comparisons of fatigue during initial treatment indicated that women who received chemotherapy reported greater fatigue severity and disruptiveness than women receiving radiotherapy. Women not pre-treated with chemotherapy experienced increased fatigue over the course of radiotherapy. Results confirmed predictions that fatigue in women with early stage breast cancer differs as a function of the type of treatment and sequencing of treatment. Findings indicating increases in fatigue during radiotherapy only among women not pretreated with chemotherapy suggest a response shift, or a change in internal standards, in women's perceptions of fatigue as a function of prior chemotherapy treatment.