The effectiveness of computed body tomography (CT) in the workup, treatment planning, and follow-up of 38 patients with Hodgkin's disease and 59 with non-Hodgkin's lymphoma was analyzed. CT scanning can frequently define lymphoma in the retroperitoneum, and occasionally in mesenteric lymph nodes, spleen, and liver. These data are useful for staging, for radiotherapy treatment planning, and in monitoring response to radiotherapy or chemotherapy. CT was found to be particularly useful in patients with large mediastinal masses. Analysis of patterns of intrathoracic spread allowed modification of treatment techniques in 60% of patients with spread along the chest wall, in order to reduce the volume of normal tissue irradiated, while obtaining adequate dose distributions within the tumor volume. It is anticipated that chest CT scanning in lymphoma patients will lead to improved tumor control and reduction of radiation complications.