Repeated regional measurements of pulmonary blood flow and ventilation were performed in 25 patients irradiated for breast cancer. Patients were studied before and at varying times after (16 to 47 days) the start of radiotherapy. At the same time, chest radiographs were taken and symptoms recorded. Of all the parameters measured, blood flow showed the earliest and greatest decrease. Since changed in blood flow at 60 days are predictive of the changes at 300 days, we believe that earlier studies during the course of irradiation could be useful in predicting long-term effects.