INTRODUCTION:Fatigue after treatment for breast cancer (BC) is common, but poorly understood. We examined the fatigue levels during first year after radiotherapy (RT) according to the extent of RT (local or locoregional), hormonal therapy (HT) and chemotherapy (CT). The impact of comorbidity was also explored. Moreover, we compared fatigue levels in patients with the general population (GenPop) data. MATERIAL AND METHODS:BC patients (n = 250) referred for post-operative RT at St. Olavs Hospital, Trondheim, Norway, were enrolled. Fatigue was measured by the EORTC QLQ-C30-fatigue subscale, ranging from 0 to 100, before RT (baseline), after RT, and at three, six, and 12 months. Clinical and treatment-related factors were recorded at baseline. GenPop data was available from a previous survey (n = 652). Linear mixed models and analysis of covariance were applied. RESULTS:Compliance ranged from 87% to 98%. At baseline, mean value (SD) of fatigue in BC patients was 26.8 (23.4). The level increased during RT (mean change 8.3, 95% CI 5.5-11.1), but declined thereafter and did not differ significantly from pre-treatment levels at subsequent time points. In age-adjusted analyses, locoregional RT accounted for more overall fatigue than local RT (mean difference 6.6, 95% CI 1.2-12.0), but the association was weakened and not statistical significant when adjusting for CT and HT. Similar pattern was seen for CT and HT. The course of fatigue differed significantly by CT (p < 0.001, interaction test). At baseline, fatigue levels were higher in patients with than without CT, but at subsequent time points similar levels were evident, indicating a temporary adverse effect of CT. Comorbidity was significantly associated with increased level of fatigue, independent of other factors (mean difference 8.1, 95% CI 2.2-14.1). BC-patients were not significantly more fatigued than GenPop, except for immediately after ending RT, and then only among those without comorbidity (mean 35.9 vs. 25.8, p < 0.001). CONCLUSION:Comorbidity seems to be a more important determinant for fatigue levels than the cancer treatment.