One hundred and four computed tomography (CT) examinations were performed on 72 consecutive radiotherapy patients with pelvic malignancies. Prostate, bladder, rectosigmoid, gynecologic, and lymphomatous primaries were studied. Computed tomography demonstrated tumor volumes larger than clinically expected in 40% and tumor extension into unsuspected tissues in 43% Based on CT, staging was revised upward in 25%. Computed tomography aided initial treatment planning in 61%. In 27 patients having post-therapy scans, changes in tumor volume were demonstrated in 59%. Modifications in therapy were made for 30% of the follow-up group. Computed tomography is recommended as an important adjunct in the workup and management of radiotherapy patients with pelvic malignancies.