Purpose: The After Mapping of the Axilla: Radiotherapy or Surgery? (AMAROS) phase III study compares axillary lymph node dissection (ALND) and axillary radiation therapy (ART) in early breast cancer patients with tumor-positive sentinel nodes. In the ART arm, the extent of nodal involvement remains unknown, which could have implications on the administration of adjuvant therapy. In this preliminary analysis, we studied the influence of random assignment to ALND or ART on the choice for adjuvant treatment. Patients and Methods: In the first 2,000 patients enrolled in the AMAROS trial, we analyzed the administration of adjuvant systemic therapy. Multivariate analysis was used to assess variables affecting the administration of adjuvant chemotherapy. Adjuvant therapy was applied according to institutional guidelines. Results: Of 2,000 patients, 566 patients had a positive sentinel node and were treated per random assignment. There was no significant difference in the administration of adjuvant systemic therapy. In the ALND and ART arms, 58% (175 of 300) and 61% (162 of 266) of the patients, respectively, received chemotherapy. Endocrine therapy was administered in 78% (235 of 300) of the patients in the ALND arm and in 76% (203 of 266) of the patients in the ART arm. Treatment arm was not a significant factor in the decision, and no interactions between treatment arm and other factors were observed. Multivariate analysis showed that age, tumor grade, multifocality, and size of the sentinel node metastasis significantly affected the administration of chemotherapy. Within the ALND arm, the extent of nodal involvement remained not significant in a sensitivity multivariate analysis. Conclusion: Absence of knowledge regarding the extent of nodal involvement in the ART arm appears to have no major impact on the administration of adjuvant therapy. © 2009 by American Society of Clinical Oncology.