Immune checkpoint inhibitor (ICI)-based combinations have become the first-line standard of care in metastatic renal cell carcinoma (mRCC), but their activity on the primary tumor is still one of the most debated issues.The aim of our analysis was to evaluate the primary tumor's response to first-line therapy with cabozantinib or nivolumab+ipilimumab, and its correlation with metastatic response and with patient outcomes.Sixty-seven mRCC patients met the criteria for inclusion in the final analysis (30 treated with cabozantinib and 37 with nivolumab+ipilimumab). In the overall population, the primary tumor control rate (PTCR) was 90.9%; no complete responses (CR) were achieved. A significant correlation was found between the baseline size of the primary tumor's longest diameter and its response according to RECIST v1.1 criteria at the time of the second radiological assessment (rs = -0.351; P = .049). Moreover, a significant correlation between the type of primary tumor response and the response of the metastases was observed in the overall population (rs = 0.50; two-sided P < 0.001). There was also a significant correlation between primary tumor response and 1-year survival rate (P = .002), even when adjusted for the IMDC prognostic group and type of therapy (HR = 8.70; 95%CI, 2.52-30.05; P = .001).Extension of the primary tumor did not affect patient survival, while its response was significantly related to the response on metastatic disease and survival. No significant differences in terms of primary tumor shrinkage were identified between treatment with nivolumab+ipilimumab or cabozantinib in this cohort.