One major limitation of three-dimensional conformal radiation therapy that has not been adequately addressed is respiration-induced organ motion. During respiration, tumors in the abdomen can typically move from 1 to 3 centimeters. Because the size and shape of external radiation treatment fields do not change during treatment, the field size of the x-ray beam must be enlarged to encompass the tumor through the entire respiration cycle. Several manufacturers are developing respiratory gating systems. These systems allow the selective delivery of absorbed doses to moving target volumes in the abdomen during time intervals when the target volume is within the intended location. Before respiratory gated radiotherapy can be implemented clinically, the efficacy of the procedure must be justified. The magnitude of dosimetric and geometric uncertainties associated with respiratory motion must be identified to determine if gating can provide an advantage over conventional treatment techniques. In addition, clinical situations and specific types of cancer that could benefit from respiratory gating must also be identified.