The onset of fever and pulmonary infiltrates in patients who have been treated with thoracic irradiation is a relatively common occurrence. Radiation pneumonitis and infectious pneumonitis share many clinical features. The major objective of the clinician in evaluating patients who present with a febrile pneumonitis syndrome is to establish the correct diagnosis with as much certainty as possible. This article will review the pathogenesis, histopathology, and clinical features of radiation pneumonitis. Certain clinical and radiographic aspects of radiation pneumonitis will help the clinician arrive at the correct diagnosis. The patient with radiation pneumonitis will present with an insidious onset of dyspnea, fever, and nonproductive cough. Review of the chest radiograph at presentation and all chest radiographs since the completion of radiation therapy will provide the key to the clinical diagnosis of radiation pneumonitis. The infiltrate characteristically has a sharp margin that conforms to the port of irradiation. If the chest radiograph is not characteristic, it will be necessary to rule out infection. If noninvasive studies are nondiagnostic, then bronchoscopy may be necessary to exclude potential infectious agents.