Because determining cardiac output is difficult in premature infants, little is known about the hemodynamic effects of PEEP in this age group. We used pulsed-wave Doppler echocardiography to assess the hemodynamic effects of PEEP increments in ten premature infants with respiratory distress syndrome. At a PEEP of 4 cm H2O, the systemic and pulmonary blood flow and the right (SVRV) and left (SVLV) ventricular stroke volume decreased slightly, while the systemic vascular resistance (Rs) increased slightly. At a PEEP of 8 cm H2O, cardiac performance was impaired significantly, with a profound decrease of the systemic and pulmonary blood flow, SVRV and SVLV and a reflectory increase of the Rs. Surprisingly, the heart rate and mean arterial BP remained constant. The effective left ventricular afterload, which takes into account the changing intrathoracic pressure, increased slightly with increasing PEEP.