Single-breath carbon monoxide transfer factor (DLCO) and diffusion coefficient (KCO) have been compared with vital capacity and total lung capacity in 119 subjects with scoliosis, 74 with idiopathic scoliosis, 32 with congenital osteogenic scoliosis and 13 with paralytic scoliosis. In each group mean values of DLCO were below, and for KCO above, predicted values. Furthermore, in each group, DLCO was correlated with lung volume and KCO inversely correlated with lung volume. The reduction in DLCO is probably due to the reduction in the total alveolar surface area and the increased values for KCO probably result from an increase in pulmonary capillary blood volume per unit alveolar volume at low lung volumes.