The slope of alveolar plateau for nitrogen derived from the single-breath test is useful to assess the function of bilateral lung grafts, but this technique is not applicable to patients with single lung grafts due to the confounding influence of the native lung. We tested the hypothesis that the nitrogen slope measured in lateral decubitus with the graft in non-dependent position may reflect primarily the distribution of ventilation in this lung. Fifteen patients with single-lung transplantation for emphysema, 10 healthy controls, and 7 patients with advanced emphysema performed single-breath washouts in right and left lateral decubitus; nitrogen slope was measured between 75 and 100% of expired volume. In 10 transplant recipients, the volume of each lung was measured in the two postures by computerized tomography. Nitrogen slope was unaffected by posture in normal controls and emphysema patients. On the other hand, nitrogen slope in transplant recipients was invariably smaller with the graft in non-dependent than in dependent position. Values of nitrogen slope with the graft in non-dependent position were similar to those obtained in normal controls but significantly smaller than those obtained in emphysema patients. Computerized tomography studies in this position indicated that the volume expired below functional residual capacity was exclusively contributed by the graft. We conclude that in patients with single-lung transplantation for emphysema 1) measuring nitrogen slope in lateral decubitus allows to distinguish between the graft and the native lung; and 2) nitrogen slope obtained with the graft in non-dependent position reflects ventilation distribution in this lung.