BACKGROUND AND PURPOSE: Currently available minimally invasive renal tumor-ablation procedures include cryotherapy, radiofrequency ablation, and microwave thermotherapy. In this study, we investigated the ability of these three approaches to destroy experimental renal tumors in rabbits. The mechanism of potential tumor metastasis was also explored. MATERIALS AND METHODS: The VX-2 tumor line is an aggressive rabbit epidermoid tumor with a high metastatic potential. An initial experiment comparing cooled-tip microwave thermotherapy with cryotherapy and radical nephrectomy for treatment of small VX-2 tumors revealed that all microwave-treated rabbits had local recurrence and that several also had diffuse intraperitoneal carcinomatosis. In view of these results, a second experiment was performed in which 45 New Zealand White rabbits were implanted laparoscopically with VX-2 xenografts underneath the kidney capsule and divided into five groups of 9 each. The test groups were microwave thermotherapy with a 3.5-mm cooled-tip probe, microwave thermotherapy with a 3.5-mm noncooled- tip probe, radiofrequency ablation with a 1.5-mm cooled-tip probe, radiofrequency ablation with a 1.5- mm non-cooled tip probe, and cryotherapy with a 2.3-mm cryoprobe. The control groups were five rabbits that were not treated, five rabbits with tumors that had the tumor pierced with a probe but were untreated, and five rabbits that underwent nephrectomy after piercing of the tumor. Treatment was initiated 5 days after tumor implantation. One month later, all animals were euthanized and autopsied. RESULTS: At 5 days after tumor implantation, laparoscopic inspection revealed no visible peritoneal metastases. At 1 month, in the cooled and non-cooled microwave-thermotherapy groups, carcinomatosis occurred in five and six of nine animals, respectively. In comparison, carcinomatosis was detected in two of nine animals in the cryotherapy group at autopsy. With respect to cooled and non-cooled radiofrequency ablation, carcinomatosis was observed in four of nine rabbits in each group. In the control groups, none of the animals with unpierced tumors exhibited carcinomatosis, while carcinomatosis was seen in two of the five rabbits with tumor violated by piercing and in three of the five rabbits that underwent immediate nephrectomy after piercing of the tumor. CONCLUSION: Carcinomatosis occurred most frequently in animals treated with microwave thermotherapy, followed by radiofrequency ablation, and lastly cryoablation. The simple act of piercing a highly aggressive tumor can result in local spread. More disconcerting, and less well understood, is why certain ablative modalities appear to increase the rate of intraperitoneal spread.