To study the effects of upper mantle radiation therapy on pulmonary function, forced expiratory volume in one second (FEV1), vital capacity (VC), inspiratory capacity (IC), diffusing capacity for CO (DLCO) and diffusion per unit of alveolar volume (DL/VA were determined in 28 patients with Hodgkin's disease, stages 1--3, before therapy and at regular intervals thereafter. Within the first year of follow-up there were significant declines in DLCO, VC, and IC, whereas there were no significant changes in FEV1 or DL/VA. DLCO showed the greatest decline in the largest number of subjects (22/28). Eleven of the 22 had 20 to 60 percent decline of DLCO from baseline. The maximum mean decline in DLCO was -12.7 +/- 3 percent at the 87th +/- 3 days from initiation of therapy postradiation sustained through the 150th day and improving to pretreatment value (+/- 5 percent) by the 8th to 12th month. The changes in DLCO seemed to be independent of the radiation dose ranges evaluated, clinically apparent intrathoracic lymphoma, postradiation radiographic abnormalities and respiratory symptoms. We concluded that impairment in diffusing capacity and loss of vital capacity will develop in most patients receiving upper mantle radiation therapy, indicating that pulmonary reaction occurs despite lung shielding. The functional losses were prolonged and occasionally severe, but were transient and subclinical in most but not all cases. A case of fatal radiation pneumonitis affecting the lung beyond the field of irradiation is reported.