Bronchoscopic lung-volume reduction with the use of one-way endobronchial valves is a potential treatment for patients with severe emphysema. To date, the benefits have been modest but have been hypothesized to be much larger in patients without interlobar collateral ventilation than in those with collateral ventilation.We randomly assigned patients with severe emphysema and a confirmed absence of collateral ventilation to bronchoscopic endobronchial-valve treatment (EBV group) or to continued standard medical care (control group). Primary outcomes were changes from baseline to 6 months in forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), and 6-minute walk distance.Eighty-four patients were recruited, of whom 16 were excluded because they had collateral ventilation (13 patients) or because lobar segments were inaccessible to the endobronchial valves (3 patients). The remaining 68 patients (mean [A,A+-SD] age, 59A,A+-9 years; 46 were women) were randomly assigned to the EBV group (34 patients) or the control group (34). At baseline, the FEV1 and FVC were 29A,A+-7\% and 77A,A+-18\% of the predicted values, respectively, and the 6-minute walk distance was 374A,A+-86 m. Intention-to-treat analyses showed significantly greater improvements in the EBV group than in the control group from baseline to 6 months: the increase in FEV1 was greater in the EBV group than in the control group by 140 ml (95\% confidence interval [CI], 55 to 225), the increase in FVC was greater by 347 ml (95\% CI, 107 to 588), and the increase in the 6-minute walk distance was greater by 74 m (95\% CI, 47 to 100) (P<0.01 for all comparisons). By 6 months, 23 serious adverse events had been reported in the EBV group, as compared with 5 in the control group (P<0.001). One patient in the EBV group died. Serious treatment-related adverse events in this group included pneumothorax (18\% of patients) and events requiring valve replacement (12\%) or removal (15\%).Endobronchial-valve treatment significantly improved pulmonary function and exercise capacity in patients with severe emphysema characterized by an absence of interlobar collateral ventilation. (Funded by the Netherlands Organization for Health Research and Development and the University Medical Center Groningen; Netherlands Trial Register number, NTR2876.).