Abstract Background This study explores the incidence of patient-reported major toxicity—symptoms rated “moderate,” “severe,” or “very severe”—for chemotherapy regimens commonly used in early breast cancer. Patients and Methods Female patients aged 21 years or older completed a validated Patient-Reported Symptom Monitoring instrument and rated 17 symptoms throughout adjuvant or neoadjuvant chemotherapy. Fisher's exact tests compared differences in percentages in symptom ratings, and general linear regression was used to model the incidence of patient-reported major toxicity. Results In 152 patients, the mean age was 54 years (range, 24–77), and 112 (74%) were white; 51% received an anthracycline-based regimen. The proportion of patients rating fatigue, constipation, myalgia, diarrhea, nausea, peripheral neuropathy, and swelling of arms or legs as a major toxicity at any time during chemotherapy varied significantly among four chemotherapy regimens (p < .05). The mean (SD) number of symptoms rated major toxicities was 6.3 (3.6) for anthracycline-based and 4.4 (3.5) for non-anthracycline-based regimens (p = .001; possible range, 0–17 symptoms). Baseline higher body mass index (p = .03), patient-reported Karnofsky performance status ≤80 (p = .0003), and anthracycline-based regimens (p = .0003) were associated with greater total number of symptoms rated major toxicities (alternative model: chemotherapy duration, p < .0001). Twenty-six percent of dose reductions (26 of 40), 75% of hospitalizations (15 of 20), and 94% of treatment discontinuations (15 of 16) were in anthracycline-based regimens. Conclusion Capturing multiple toxicity outcomes throughout chemotherapy enables oncologists and patients to understand the range of side effects as they discuss treatment efficacies. Continuous symptom monitoring may aid in the timely development of interventions that minimize toxicity and improve outcomes.