Background: Hematologic toxicities are one of the greatest challenges in adjuvant chemotherapy for breast cancer. This analysis of the ADEBAR trial aims to evaluate application and effect of granulocyte colony-stimulating factor (G-CSF) and epoetin alfa (EPO) on hematologic parameters and fatigue in patients with breast cancer during chemotherapy. Patients and Methods: In the ADEBAR trial, 1493 patients with node-positive primary breast cancer were randomized to either 6 × 5-fluorouracil, epirubicin, and cyclophosphamide (FEC120) or 4 × epirubicin and cyclophosphamide followed by 4 × docetaxel (EC-DOC). Co-medication with G-CSF or EPO was applied to treat chemotherapy-induced leukopenia or anemia. Fatigue was assessed at baseline and after one-half of the chemotherapy. Results: In total, 899 patients could be included in the analysis. There was no evidence for an association between leucocyte or hemoglobin levels and application of G-CSF and EPO in the preceding cycle, respectively. Hemoglobin levels (B = −0.41; P < .001) were affected by treatment regimen. Fatigue during chemotherapy was mostly affected by the level of fatigue before the start of chemotherapy (B = 0.41; P < .001). Patients with G-CSF application in the preceding cycle showed an increased fatigue score (B = 5.43; P = .02). Conclusion: We showed that fatigue during adjuvant chemotherapy was mostly affected by the level of fatigue present before the start of chemotherapy. This result suggests that the level of fatigue before the start of treatment should be included as an important factor when deciding on type and toxicity of chemotherapy in early breast cancer. This analysis of the ADEBAR trial evaluated the effect of granulocyte colony-stimulating factor and epoetin on hematologic toxicity and fatigue in 899 patients with high-risk early breast cancer during chemotherapy. The results indicate that fatigue was affected mostly by the level of fatigue before the start of therapy.