Three patients with pericarditis following therapeutic irradiation for malignant tumors were studied at autopsy. In one subject, death occurred five days after the last dose of radiation. The pericardial involvement was characterized by fibrinous exudation. This was more marked in degree over the right side of the heart than the left and corresponded with the field of irradiation. In the remaining two patients, two and four courses of radiation had been applied, the interval between the last dose and death being seven weeks in one patiet and 1 1/2 years in the other. The lesions were those of organizing pericarditis.