To investigate the usefulness of bronchoscopic examination, computerized tomography (CT), and radionuclide quantitative ventilation-perfusion lung scan to determine the response of patients with obstructive endobronchial tumors to photodynamic therapy (PDT), the findings in 24 patients treated with PDT were analyzed. PDT was found to be most effective when the tumor was bronchoscopically polypoid in appearance, with little or no submucosal invasion or peribronchial extension seen on CT scans. With increasing submucosal and/or peribronchial extension, the immediate response to treatment was poorer. Patients who had 50% or more of the airway obstruction due to mucosal tumor had no evidence of local tumor recurrence for a median interval of 22 weeks after treatment. In patients with predominant submucosal and/or peribronchial tumor, the duration of response was 7 weeks. Response to treatment did not correlate with the degree of airway obstruction. CT provided valuable information regarding the extent of the peribronchial involvement and airway distortion, which was often underestimated by bronchoscopy alone. Absent perfusion or reduction of regional perfusion out of proportion to ventilation on scintigraphy in the involved lung zone was found to be associated with extensive peribronchial involvement and a poor outcome. The authors conclude that the addition of CT and radionuclide quantitative ventilation-perfusion lung scan to bronchoscopic examination is useful in predicting the response of patients with obstructive endobronchial tumors to PDT.