Key PointsQuestionCan longitudinal changes in systemic inflammatory response, measured by C-reactive protein and albumin within the modified Glasgow prognostic score (mGPS), predict response and outcomes in patients with metastatic renal cell carcinoma?FindingsIn this prognostic study, which was a post hoc analysis of the IMmotion150 trial (915 patients) and IMmotion151 trial (305 patients), longitudinal measurement mGPS during treatment provided valuable prognostic information regardless of imaging-assessed treatment response. In the disease control subgroup, on-treatment mGPS exhibited superior and independent prognostic information compared with radiologic staging alone.MeaningThese findings suggest that integrating on-treatment mGPS for more holistic and patient-centered therapy monitoring in addition to radiologic staging may improve clinical care at a low cost for patients with metastatic renal cell carcinoma.