Purpose: To evaluate the feasibility, safety, and efficacy of retroperitoneoscopic microwave ablation (MWA) for clinical stage T-1a (cT(1a)) renal tumors. Patients and Methods: Consecutive renal cell carcinomas (RCCs) managed since April 2006 with a minimal follow-up of 12 months were included. A total of 23 tumors in 22 patients were managed with laparoscopic MWA. A cooled shaft needle antenna was inserted into the tumor under direct visual guidance. Microwaves were emitted at 50W for 8 minutes and prolonged as necessary to ensure complete tumor kill. Short-term efficacy was assessed by contrast-enhanced CT at 1, 3, and 6 months, and every 6 months thereafter. Results: Pathologic analysis revealed low-grade clear-cell RCC in all patients. Mean tumor size was 2.8 cm (range 0.9-4.0 cm). Excluding 5 lost patients, the initial ablation was successful in 17 (94.4%) of 18 tumors. One patient with an incomplete ablation lesion under strict surveillance had no evidence of disease progression at 31 months of follow-up. No recurrence was observed at a median follow-up of 20 months (range 12-45 mos). All 17 remaining patients had no clinical or radiographic evidence of disease at last follow-up. In addition, complications were mild and tolerable (18.2%), and there was no significant deterioration of renal function. Conclusions: Retroperitoneoscopic MWA appears to be a safe and effective technique for cT(1a) RCC in selected patients. Additional follow-up is needed to assess long-term effectiveness.