After single-lung transplantation (SLT) for emphysema, heterogeneity of ventilation distribution in the graft can be assessed by measuring the slope of the alveolar plateau, computed from a single-breath test, performed in lateral decubitus with this lung in the nondependent position. We tested the validity of this technique in patients with SLT for interstitial lung diseases (ILD). Twelve patients with SLT for ILD, 12 nontransplanted patients with ILD, and 10 healthy control subjects performed single-breath washouts in right and left lateral decubitus; nitrogen slope (SN 2 ) and the difference between SF6 and He slopes (SSF 6 -SHe) were measured between 75 and 100% of expired volume. In 10 transplant recipients, the volume of each lung was measured in both postures by computerized tomography. Slopes were unaffected by posture in normal control subjects and patients with ILD. On the other hand, SN 2 and SSF 6 -SHe in transplant recipients were smaller with the graft in the nondependent than in the dependent position (0.366 Ϯ 0.445 vs. 1.035 Ϯ 0.498 for SN 2 ; 0.094 Ϯ 0.201 vs. 0.218 Ϯ 0.277 for SSF 6 -SHe). Values of SN 2 and SSF 6 -SHe obtained in the former position were similar to those obtained in normal controls, while values obtained in the latter position were similar to those obtained in nontransplanted patients with ILD. Computerized tomography studies with the graft in the nondependent position indicated that this lung contributed 82% of the volume expired below functional residual capacity. We conclude that, in patients with SLT for ILD, the slope of the alveolar plateau obtained with the graft in the nondependent position reflects heterogeneity of ventilation distribution in this lung. lung transplantation; pulmonary fibrosis; distribution of ventilation PREVIOUS STUDIES AFTER BILATERAL or heart-lung transplantation have shown that the heterogeneity of ventilation distribution in the allograft is increased by infection (9) and by acute and chronic rejection (2, 5, 8 -10, 12). For example, the slope of the alveolar plateau for nitrogen or helium obtained during a single-breath washout test increases in the early stages of bronchiolitis obliterans, which is considered a manifestation of chronic allograft rejection and is characterized by gradual obliteration of the small airways (3). Based on these studies, measuring the slope of the alveolar plateau has been advocated as a noninvasive tool for the early detection of this complication in recipients of bilateral grafts (2, 5, 10, 12).When performed in the seated posture in patients with single-lung transplantation, the single-breath test cannot be used to assess the distribution of ventilation in the allograft, because both the native and the transplanted lungs contribute to the slope of the alveolar plateau. However, our laboratory recently showed in patients with single-lung transplantation for emphysema that this limitation can be overcome by performing the test in lateral decubitus (11). When the patient was in lateral decubitus with the graft in th...