Stereotactic body radiation therapy (SBRT) is currently under active study at numerous centers for clinical application in the management of patients with primary or metastatic tumors of the liver, primary or metastatic tumors of the kidney, and selected other retroperitoneal tumors. Accurate patient positioning and tumor relocalization are essential for SBRT use in the liver and other abdominal and retroperitoneal sites, as at other tumor sites. In a phase I clinical trial at the University of Colorado, patients with liver metastases have received SBRT. Eligible patients had 1-3 discrete liver metastases and no prior radiotherapy to the liver. The aggregate tumor diameter (sum of diameters) was <6 cm. Respiratory control was used. Normal liver volume to be preserved was determined prior to therapy. Dose was prescribed to a planning target volume that included the gross tumor volume plus at least a 5-mm radial and 10-mm superior-inferior margin. SBRT was administered with 6- to 15-MV beams through either dynamic conformal arcs or a combination of multiple noncoplanar static beams. The dose was safely escalated to 60 Gy in 3 fractions. After SBRT to hepatic lesions, it is extremely difficult to radiographically evaluate tumor response within the first few months, and radiographic response analysis may require 4-6 months after SBRT. Care must be taken to avoid focal high-dose therapy to the gastrointestinal mucosa, where the maximum point dose is likely to be the major limitation rather than the mean dose. SBRT has a potential role in the management of renal cell carcinoma, either as an alternative to surgery to the primary site or as cytoreductive therapy directed toward metastatic sites, and in the management of selected retroperitoneal tumors.