Both single lung transplantation and bilateral lung transplantation have been successful in patients with chronic obstructive pulmonary disease. Limited availability of donor lungs dictates wider use of single lung transplantation as long as this procedure provides a comparable functional result at less risk. Early morbidity, mortality rates, and functional results were compared in 32 consecutive patients undergoing either single or bilateral lung transplantation for end-stage chronic obstructive pulmonary disease. When pretransplantation and posttransplantation values of forced expiratory volume in 1 second, single breath diffusing capacity, total lung capacity, arterial oxygen tension, and 6-minute walking distance were compared, statistically significant improvement (p less than 0.01) was seen in both groups. However, significantly higher values (p less than 0.01) were attained for forced expiratory volume in 1 second, single breath diffusing capacity, and arterial oxygen tension in the bilateral transplantation group than in the single lung transplantation group. Patients receiving bilateral lung transplants were at greater risk for postoperative complications, especially cardiac arrhythmias and bronchial anastomotic defects. Actuarial survival for the single lung transplantation group was 93% at 1 year, versus 87% at 6 months and 71% at 1 year for the bilateral lung transplantation group. The optimal transplantation procedure for patients with chronic obstructive pulmonary disease has not been determined. Longer follow-up periods are needed to define better the roles of bilateral and single lung transplantation in this group of patients.