Regional and overall lung function was studied in 14 women during a one-year follow-up after postmastectomy irradiation for cancer of the breast. The dose was 40 Gy in the chest wall and 20-40 Gy in the apex of the lung. Immediately after irradiation there was a slight increase in pulmonary clearance of 99TCm-DTPA compared to baseline values. This increase, however, was not restricted to the field of irradiation. Three months after irradiation there were regional defects in the ventilation and perfusion scintigrams in 11 of the patients. At the same time there was a greater decrease in total lung capacity, vital capacity and diffusion capacity for CO than in ventilation and perfusion. One year after irradiation the lung function impairment had only insignificantly regressed. It is concluded that local lung irradiation may cause persistent regional and generalized lung function impairment. The early increase in pulmonary clearance of 99Tcm-DTPA could not be used as a predictor of subsequent lung function impairment.