In this paper, we report the characterization of microwave therapy in normal porcine kidneys both in vitro and in vivo. This technology is being developed for eventual use in the treatment of small renal cell carcinomas (RCC) using minimally invasive procedures. Microwave energy was applied through an interstitial microwave probe (Urologix, Plymouth, MN) to the kidney cortex with involvement of the medulary region. The thermal histories at several locations were recorded. After treatment, the kidneys were bisected and tissue sections were prepared for histologic study at approximately the same depth as the thermal probe. Histologic cellular injury and microvascular stasis were quantitatively evaluated. Absolute rate kinetic models of cellular injury and vascular stasis were fit to the thermal and histologic data to determine the kinetic parameters. A 3-D finite element thermal model based on the Pennes Bioheat equation was developed and solved using a commercial software package (ANSYS, V5.7). The specific absorption rate (SAR) of the microwave probe was measured experimentally. This is the first thermal model validated using measured in vitro thermal histories and then used to determine the blood perfusion term in vivo.