One hundred forty-five patients with hepatoma had CT scanning for radiation therapy treatment planning. In order to demonstrate the anatomical distortions that occur with hepatoma and its effect on treatment planning, a control group of 50 colorectal cancer patients with normal livers was analyzed for comparison. The objectives of planning were to deliver as homogeneous a dose to the whole liver as possible and not to treat more than one of two functional kidneys or more than one-half of both functional kidneys. Conventional AP/PA portals were defined by physical examination, intravenous pyelogram, and bowel gas patterns at simulation and were found to be inadequate for the treatment of 76% of patients with hepatoma and 10% of patients with normal livers. Among the control group patients with no hepatoma, only 10% required oblique portals and 6% could not be treated because of left hydronephrosis or a solitary right kidney. Because the distortion of the liver in hepatoma in relationship to the kidneys required portal modification in 76% of hepatoma cases; 39% required oblique planning, 24% AP/PA, 20% PA and left lateral portals, and 17% required 4-field, 3-field or other plans in order to meet the treatment planning objectives. We concluded that all patients receiving radiation therapy to the liver for hepatoma require CT scanning for optimum radiation therapy treatment planning because of the hepatic distortion that occurs in hepatoma and the requirements of renal tolerance.