High doses of external beam radiotherapy are required to obtain local control of many intrathoracic neoplasms. Because spinal cord tolerance limits the radiation dose that can be given through anterior and posterior fields, it is often necessary to increase the dose to the primary tumor site using oblique or lateral fields. When pulmonary infiltrates develop following treatment in these patients, it is frequently difficult to distinguish between infection, recurrent tumor, and radiation pneumonitis. In nine patients in whom acute pulmonary infiltrates occurred following a course of thoracic irradiation, computed tomography (CT) with computed dosimetric reconstruction (CDR) were studied in an attempt to correlate the treatment volumes with the location and configuration of the infiltrates and in this way establish the source for the pulmonary abnormalities. In seven of these patients, the diagnosis was changed, and the resulting post-irradiation clinical therapy was altered as a consequence of this retrospective study.