We prospectively studied patients with lung carcinoma and borderline lung functions (forced expired volume [FEV1] of less than 1500 ml or a dyspnoea score of 2  at presentation), who were treated with high dose irradiation. Patients were divided into those with suprahilar and hilar tumours. Lung perfusion was assessed in upper, middle and lower zones for each lung at presentation. The ipsi-lateral upper and middle zone were regarded as at risk from irradiation in patients with suprahilar tumors and the whole ipsi-lateral lung in patients with hilar tumors. Lung function was measured at presentation (18 patients) at 4-6 month follow up (16 evaluable patients = group 1) and again at 10-12 month follow up (10 evaluable patients = group 2). A worsening of the dyspnoea score (3 in group 1 and 2 in group 2) occurred only in patients with a greater than 10% decrease in transfer factor irrespective of the change in FEV1. A statistically significant correlation was found between decreased transfer factor at follow up and the perfusion in the lung zones regarded as at risk from irradiation at presentation (Spearman's rank correlation). There was no correlation between perfusion and changes in the FEV1. Patients in whom lung perfusion was less than 35% in the zones at risk tended not to have decreased transfer factor at follow up. These findings indicate that worsening in the patients' dyspnoea score after irradiation is dependent on decreased transfer factor rather than FEV1 and that patients with borderline lung functions may be treated with irradiation if the perfusion in the zones at risk from radiation is less than 35%.