OBJECTIVES: To prevent cancer recurrence, each year over 100,000 US women begin a 5-year course of aromatase inhibitors (AIs) for early-stage breast cancer. AI-related joint pain (arthralgia) may interfere with patients’ well-being in several domains. There is no arthralgia measurement tool validated in this population specifically, and more information is needed about the impact of arthralgia on patient-reported outcomes (PROs) in these patients. We sought to assess relationships of arthralgia with PROs over the 1st 12 weeks of AI therapy. METHODS: Postmenopausal female oncology outpatients with early stage breast cancer (N=52, pilot sample of the Breast Cancer Adjuvant Therapy cohort) completed paper surveys prior to AI initiation, and every 2 weeks thereafter for 12 weeks. Pain was measured in 16 joint locations using a 0-10 numeric rating scale. Baseline covariates included age, comorbidities, existing major depressive disorder (PHQ-2), social support (DUFSS), performance status (ECOG), and menopausal symptoms (FACT-ES). Time-varying PROs examined included physical function, sleep disturbance, pain interference, and emotional distress (depression), all of which were measured using PROMIS static short forms. We used mixed models to analyze arthralgia and PROs. RESULTS: Mean age was 62 years (SD=10). The majority of women had active performance status (n=46) and no major depression (n=47) at baseline. Median worst pain in any joint prior to AI initiation was 1 out of 10 (interquartile range=0-5). Adjusting for baseline covariates, greater arthralgia was associated with worse physical function (β=-0.09[-0.13,-0.04]), greater pain interference (β=0.15[0.09,0.20]), and greater emotional distress (β=0.06[0.01,0.11]) but not with sleep disturbance (β=-0.01[-0.03,0.004]). CONCLUSIONS: These preliminary findings will be used to develop and validate the Patient-Reported Arthralgia Inventory, toward improving arthralgia measurement. Our findings contribute to understanding how arthralgia relates to PROs over the 1st 12 weeks of AI therapy. Targeted AI adherence interventions will rely on comprehensive longitudinal PRO information.