Abstract Background: Most early-stage breast cancer (EBC) patients (pts) do not experience signs or symptoms of disease; approximately 90% of women diagnosed in breast screening are asymptomatic in the US (Ryerson et al. 2015). Rather, side effects of cancer therapy have the greatest impact and can be burdensome to pts on and after treatment. Bother and impact have not been thoroughly assessed from the patient perspective in trials. Qualitative research with 56 pts undergoing or completing (after 3 and within 24 mos of) systemic treatment were conducted to assess the need for EBC-focused patient-reported outcome (PRO) measures. Methods: Semi-structured interviews were conducted to better understand the treatment experience; the interview guide was developed in consultation with breast cancer advocates who were former pts. The interview sample was determined to capture findings across EBC therapies (HER2-targeted [HER2], hormone/endocrine [H/E], and/or chemotherapy [CT]). Treatment experience, including treatment-related symptoms and treatment impact (e.g. on activities of daily living, emotional aspects) were discussed in each 90-minute session. Pts rated level of bother of symptoms and impacts on an 11-point scale. Disease stage, treatment received, surgery, and other health information was collected from medical charts. Qualitative analysis was conducted with ATLAS.ti software. Symptom data was reviewed to appropriately analyze therapy subgroups. Results: Stage Ia (17.9%), Ib (14.3%), IIa (32.1%), IIb (25.0%), or IIIa (7.1%) pts that received adjuvant (75%) or neoadjuvant (25%) therapy participated; 106 unique treatment-related symptoms were reported. Symptoms most frequently reported included hair loss (86.7%), change in taste (73.3%), and tiredness/fatigue (71.1%) on CT (n=45); tiredness/fatigue (34.8%), runny nose (26.1%), and watery eyes (21.7%) on HER2 (n=23); and hot flashes (50.0%), joint pain (37.5%), and weight gain (20.1%) on H/E (n=24). The most common symptoms reported after therapy completion included memory loss (63.6%), symptoms of neuropathy (numbness, tingling, and pain in fingers, 63.6%), and tiredness/fatigue (45.5%) (n=11). CT symptoms rated by ≥ 25% of pts that were most bothersome included tiredness/fatigue (x -=8.2, n=18**), hair loss (x -=8.2, n=32**), and memory loss (x -=7.7, n=15**). HER2 and H/E ratings of bother were less frequent. EBC treatment was associated with significant impact on pts' lives; categories described are below: Impact category Average bother rating* (n**)Concerns with treatment9.5 (2)Physical/functional7.7 (36)Work or school7.5 (37)Sleep7.5 (21)Daily tasks and activities7.4 (95)Emotional7.4 (62)Sexual behavior7.1 (22)Cognitive function6.8 (6)Social6.7 (54)Appearance6.5 (32)* Rating on 11-pt scale; 0=none to 10=extremely bothersome ** n=number of patients rating the level of bother Conclusion: Treatment-related symptoms and associated degree of bother differed by treatment group. Pts' descriptions of treatment impact provided additional insight into the burden of EBC. EBC-specific PROs included in trials that gain pts' perspective on experience with treatment would further inform pts and may also inform therapy choice. Citation Format: Petersen JA, Gauthier MA, Piault E, DeBusk KPA, Buzaglo JS, Eng-Wong J, Glazer JR, Green MC, Johnson JM, Spears PA, Evans CJ. Importance of the patient voice in drug development: Early-stage breast cancer and measurement gaps concerning the treatment experience. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-10-20.