Ventilation/perfusion scans were obtained in 45 patients with unresectable carcinoma of the bronchus. Of these 45 patients, 35 were reinvestigated shortly after radiotherapy and 17 of them had additional later follow-up studies. Both ventilation and perfusion were always abnormal in the lung affected by the tumor; perfusion was usually more impaired than ventilation. These abnormalities were difficult to detect or to evaluate from the standard chest radiograph. After radiotherapy, ventilation improved in 83% and perfusion in 86% of the patients. This improvement was associated with amelioration of breathlessness, which improved in 74% of the patients. Slow but progressive deterioration of regional ventilation and perfusion were subsequently observed. This was often associated with the development of radiation fibrosis. Spirometric measurements (VC, FEV1) were moderately imparied at the initial assessment (83% and 66% of predicted, respectively), probably due to coexisting chronic airway obstruction, and did not show significant changes after radiotherapy or during the follow-up. Radionuclide studies of regional perfusion and ventilation proved more sensitive and more specific than tests of overall lung function for the functional assessment and follow-up of patients with unresectable carcinoma of the bronchus.