Background: The phase 3 KATHERINE (NCT01772472) study, met its primary endpoint by demonstrating significantly improved invasive disease-free survival with adjuvant T-DM1 compared to H in pts with residual invasive disease after neoadjuvant chemotherapy plus HER2-targeted therapy. PROs are reported here. Methods: Eligible pts had HER2-positive early breast cancer, received taxane- and H-containing neoadjuvant therapy (with/without anthracyclines) followed by surgery, and had residual invasive disease in the breast and/or axillary nodes. Pts were randomized to 14 cycles of adjuvant T-DM1 (3.6 mg/kg IV q3w) or H (6 mg/kg IV q3w) and adjuvant endocrine and radiation therapy per standard of care. The EORTC Quality of Life Questionnaire–Core 30 (QLQ-C30) and QLQ–Breast Cancer (QLQ-BR23) were completed at screening, at day 1 of cycles 5 and 11, within 30 days after study drug completion, and at 6 and 12 months’ follow-up. Results: Of 1,486 pts randomized (T-DM1, n = 743; H, n = 743), 612 (82%) and 640 (86%), respectively, had valid baseline and ≥1 post-baseline PRO assessments. During the study, pts in both arms had similar mean scores on the QLQ-C30 and QLQ-BR23 function and symptom scales. There was no clinically meaningful change (≥10 points) from baseline in the mean scores in either arm, including on symptoms similar to AEs seen with T-DM1 (eg, fatigue). While more pts in the T-DM1 arm reported clinically meaningful deterioration in role functioning (49% vs 41%), appetite loss (38% vs 28%), constipation (47% vs 38%), fatigue (66% vs 61%), nausea/vomiting (39% vs. 30%), and systemic therapy side effects (49% vs 36%) at ≥1 assessment, the proportion reporting clinically meaningful change in functioning was similar between arms at any given assessment. Conclusions: Mean scores showed only small deterioration from baseline in patient-reported treatment-related symptoms in both study arms. While more pts in the T-DM1 arm reported deterioration at some point in several symptoms, baseline global health status and functioning were generally maintained in both arms over the treatment course. Clinical trial information: NCT01772472