Acute radiation pneumonitis lacks any single pathognomonic morphologic feature, but is a diagnosis made from a combination of alveolar septal thickening, proliferation and desquamation of atypical septal cells, hyaline membrane formation, and pulmonary vascular changes. The pathogenesis of radiation pneumonitis is poorly understood. Total radiation dosage, total volume irradiated, and duration of therapy are significant factors but cannot explain all cases. Seven autopsied cases of bilateral radiation pneumonitis following megavoltage therapy for lung carcinoma are reported and analyzed. The use of extensive fields, or of rotational technique, necessitated irradiation of a large volume of lung tissue. The inaccuracy of calculated dosimetry, coupled with possible errors in port placement can result in much higher bilateral radiation doses than anticipated. Infection, by increasing absorption of radiation, probably causes an increased susceptibility to radiation pneumonitis. While the relatively small doses of radiation reaching the contralateral lung are usually innocuous, certain patients are hyperreactive and respond with a severe diffuse radiation pneumonitis.