PURPOSE: Cancer-related fatigue (CRF) is a prevalent and distressing side effect of cancer and its treatment that remains inadequately understood and poorly managed. A better understanding of the factors contributing to CRF could result in more effective strategies for the prevention and treatment of CRF. The objectives of this study were to examine the prevalence, severity, and potential predictors for the early onset of CRF after chemotherapy cycle 1 in breast cancer patients. METHODS: We report on a secondary data analysis of 548 female breast cancer patients from a phase III multi-center randomized controlled trial examining antiemetic efficacy. CRF was assessed by the Brief Fatigue Inventory at pre- and post-chemotherapy cycle 1 as well as by the four-day diary. RESULTS: The prevalence of clinically relevant post-CRF was 75%. Linear regression showed that pre-treatment CRF, greater nausea, disturbed sleep, and younger age were significant risk factors for post-CRF (adjusted R(2) = 0.39; P < 0.0001). Path modeling showed that nausea severity influenced post-CRF both directly and indirectly by influencing disturbed sleep. Similarly, pre-treatment CRF influenced post-CRF directly as well as indirectly through both nausea severity and disturbed sleep. Pearson correlations showed that changes in CRF over time were significantly correlated with concurrent changes in nausea severity (r = 0.41; P < 0.0001) and in disturbed sleep (r = 0.20; P < 0.0001). CONCLUSION: This study showed a high prevalence (75%) of clinically relevant CRF in breast cancer patients following their initial chemotherapy, and that nausea severity, disturbed sleep, pre-treatment CRF, and age were significant predictors of symptom.