A 15-year-old boy with bronchiolitis obliterans after bone marrow transplantation successfully underwent bilateral living-donor lobar lung transplantation (LDLLT) with segmentectomy of the superior segment of an oversized right lower lobe graft. As the recipient was small for his age, the predicted value of his functional vital capacity of the recipient was difficult to determine preoperatively. Three-dimensional computed tomography (CT) volumetry revealed that the ratio of donor graft volume to recipient hemithorax volume was 159% on the right side and 82% on the left side. The patient is alive and well 7 months after transplantation, and three-dimensional CT volumetry revealed that the right and left donor lungs were still compressed to 73% and 84% of the original size, respectively. In LDLLT, segmentectomy of the superior segment of the lower lobe is a useful option for downsizing an oversized graft and three-dimensional CT volumetry can provide meaningful data for size matching.