Background: Greater illness acceptance is associated with lower levels of psychological distress and higher rates of completion of advance care planning in adults with advanced cancer. However, studies on factors associated with illness acceptance among older adults with advanced cancer are limited and historically have not explored its relationship with caregiver or physician characteristics, despite the important role caregivers and oncologists play in patient illness acceptance. We aimed to evaluate the associations of patient, caregiver, and physician-related factors with illness acceptance among older adults with cancer. Methods: We utilized data from a national geriatric assessment cluster-randomized trial (NCT 02054741; PI: Mohile; funding NCI UG1CA189961). We recruited patients aged 70 years or over with stage III or IV incurable solid tumors or lymphoma and at least one geriatric assessment domain impairment as well as their caregivers. Illness acceptance was assessed using the 5-item Peace, Equanimity, and Acceptance in the Cancer Experience (PEACE) questionnaire (range 5-20). Higher PEACE score indicates greater illness acceptance. Multiple linear regression was used to assess the associations of patient, caregiver, and physician-related variables with PEACE. Variables included patient sociodemographic factors, geriatric assessment variables, and estimated life expectancy; caregiver sociodemographic variables, anxiety, depression, and physical function; and oncologist years in practice and comfort with geriatrics. Results: We included 333 patient-caregiver dyads; mean age (SD) of patients and caregivers, respectively, were 76.8 (5.4) and 66.6 (12.1). Approximately 27%, 26%, and 16% of patients had lung, gastrointestinal, and genitourinary cancers, respectively. Mean PEACE score was 17.4 (2.5), indicating relatively high illness acceptance (Table). On regression analysis, patient psychological impairment domain (including both anxiety and depression) (B=-0.89; p>0.01), patient’s lower estimated life expectancy (≤1 vs. >5 years, B=-1.19, p=0.02), and lower caregiver education attainment (≤high school vs. college; B=-0.86; p<0.01) were associated with a lower illness acceptance. No physician variables were associated with PEACE. Conclusions: A better understanding of illness acceptance and its associated factors can facilitate patient-caregiver dyadic interventions to improve illness acceptance.