OBJECTIVE: To examine if a comorbid diagnosis of knee or hip osteoarthritis (OA) in older adults with new back pain visits is associated with long-term patient-reported outcomes and back-related health care use. DESIGN: Prospective cohort study. SETTING: Three integrated health systems forming the Back pain Outcomes using Longitudinal Data cohort. PARTICIPANTS: Participants (N=5155) were older adults (≥65y) with a new visit for back pain and a complete electronic health record data. INTERVENTIONS: Not applicable; we obtained OA diagnoses using diagnostic codes in the electronic health record 12 months prior to the new back pain visit. MAIN OUTCOME MEASURES: The Roland-Morris Disability Questionnaire (RDQ) and the EuroQol-5D (EQ-5D) were key patient-reported outcomes. Health care use, measured by relative-value units (RVUs), was summed for the 12 months after the initial visit. We used linear mixed-effects models to model patient-reported outcomes. We also used generalized linear models to test the association between comorbid knee or hip OA and total back-related RVUs. RESULTS: Of the 5155 participants, 368 (7.1%) had a comorbid knee OA diagnosis, and 94 (1.8%) had a hip OA diagnosis. Of the participants, 4711 (91.4%) had neither knee nor hip OA. In adjusted models, the 12-month RDQ score was 1.23 points higher (95% confidence interval [CI], 0.72-1.74) for patients with knee OA and 1.26 points higher (95% CI, 0.24-2.27) for those with hip OA than those without knee or hip OA, respectively. A lower EQ-5D score was found among participants with knee OA (.02 lower; 95% CI, -.04 to -.01) and hip OA diagnoses (.03 lower; 95% CI, -.05 to -.01) compared with those without knee or hip OA, respectively. Comorbid knee or hip OA was not significantly associated with total 12-month back-related resource use. CONCLUSIONS: Comorbid knee or hip OA in older adults with a new back pain visit was associated with modestly worse long-term disability and health-related quality of life.