Thirty-four patients receiving high-dose hemithorax irradiation as part of the treatment for pleural mesothelioma were studied with regard to changes in lung function following irradiation, and these changes were correlated with the radiologically-assessed lung injury. The latter was scored from 0 to 500 and found to be severe by 6 months (mean score 360), very severe by 9 months (mean score 430), and nearly total by 12 months (mean score 480) after treatment. Forced vital capacity and diffusing capacity both showed a significant decline at 1.5-2 months following the end of radiotherapy and thereafter up to the end of the 1 year follow-up period. Neither of these variables could be correlated consistently with the radiologically-assessed changes. Hypoxemia and pathological physiological shunting increased transiently 1-2 months after irradiation in 2 of the 6 patients monitored. The observed radiologically-assessed final effects of high-dose hemithorax irradiation are compatible with a total loss of lung function on the irradiated side. Before this form of treatment is used, lung function should be evaluated as for pneumonectomy.