A simple modification of the Isolette incubator -respirator such that a constant negative (subatmospheric) pressure (CNP) may be applied to the body chamber is described. A constant subatmospheric pressure around the thorax (constant positive transpulmonary pressure) has been used in the treatment of five newborn infants with severe hyaline membrane disease with favourable results. Arterial Po2 increased significantly while the alveolar-arterial Po2 difference decreased. There was no significant influence on arterial Pco2. This approach appears to be a useful adjunct to the treatment of hyaline membrane disease since it increases arterial Po2, allows more rapid reduction of inspired o2 concentrations to levels not toxic to the lung, and may obviate the need for endotracheal intubation and artificial respiration.