Multiple treatment options are available for clinical T1 renal cell carcinoma (cT1 RCC) including surgery, thermal ablation, and active surveillance. Prior studies of cryoablation and radiofrequency ablation recommend ablation for tumors < 3cm. However, high powered microwave ablation (MWA) has fewer technical limitations and potentially enables larger more reliable ablation zones. The purpose of this study was to evaluate the effect of tumor diameter on oncologic outcomes for patients with clinically localized cT1 RCC treated with surgery or MWA. A prospectively maintained database of patients with clinically localized RCC treated with either radical nephrectomy (RN), partial nephrectomy (PN), or MWA from 2000 to 2020 was utilized. Local recurrence-free (LRFS), metastasis-free (MFS), and cancer-specific survival (CSS) were estimated using the Kaplan-Meier method. Variables associated with survival were determined using Cox proportional hazard models (Table). 1209 patients were treated for cT1 RCC (353 MWA, 398 PN, 458 RN). Patients who underwent MWA were older (p<0.001) and had higher Charlson Comorbidity score (p<0.001). Median follow-up was 42 months (IQR 21-75).;Compared to surgery, MWA was associated with similar MFS and CSS when adjusting for age, histology, size, and grade (Table). MWA was, however, associated with an increased risk of local recurrence on multivariate analysis (HR 3.79, 95% CI 1.39-10.3). Of the 21 patients with local recurrence following MWA, 12 (57%) underwent repeat ablation (4 had surveillance, 4 had surgery, 1 had SBRT.)Stratified by size, RCC between 3.1-4 cm and 4.1-5 cm treated with MWA had no difference in LRFS compared to lesions < 3 cm treated with MWA (Figure). Lesions >5 cm treated with MWA had higher risk of local recurrence compared to lesions < 3 cm (HR 3.82, 95% CI 1.02, 14.3). MWA effectively treats cT1 RCC with similar rates of metastatic progression and cancer specific survival compared to surgery. Local recurrence is more common with MWA, but recurrences can be salvaged by a second treatment. MWA effectively treats tumors up to 5 cm without compromising recurrence outcomes, which may suggest that the 3 cm size limit be re-evaluated with MWA. [ABSTRACT FROM AUTHOR]