Background: The National Cancer Institute's Patient-Reported Outcomes Version of the Common Terminology Criteria for Adverse Events, collected alongside the clinician-reported Common Terminology Criteria for Adverse Events, enables comparisons of patient and clinician reports on treatment toxicity. Methods: In a multisite study of women receiving chemotherapy for early-stage breast cancer, symptom reports were collected on the same day from patients and their clinicians for 17 symptoms; their data were not shared with each other. The proportions of moderate, severe, or very severe patient-reported symptom severity were compared with the proportions of clinician-rated grade 2, 3, or 4 toxicity. Patient-clinician agreement was assessed via κ statistics. Chi-square tests investigated whether patient characteristics were associated with patient-clinician agreement. Results: Among 267 women, the median age was 58 years (range, 24-83 years), and 26% were nonwhite. There was moderate scoring agreement (κ = 0.413-0.570) for 53% of symptoms, fair agreement for 41% (κ = 0.220-0.378), and slight agreement for 6% (κ = 0.188). For example, patient-reported and clinician-rated percentages were 22% and 8% for severe or very severe fatigue, 41% and 46% for moderate fatigue, 32% and 39% for mild fatigue, and 6% and 7% for none. Clinician severity scores were lower for nonwhite patients in comparison with white patients for peripheral neuropathy, nausea, arthralgia, and dyspnea. Conclusions: Although clinician reporting of symptoms is common practice in oncology, there is suboptimal agreement with the gold standard of patient self-reporting. These data provide further evidence supporting the integration of patient-reported outcomes into oncological clinical research and clinical practice to improve monitoring of symptoms as well as timely interventions for symptoms.