Continuous negative extrathoracic pressure (CNEP) has been recently reintroduced as therapy for respiratory failure. To determine its effects on cardiac output a pilot study was performed in ten patients aged 2 months-3 years (median 4 months). All had chronic respiratory failure (seven with bronchopulmonary dysplasia). Five were breathing spontaneously and five were intubated and undergoing intermittent positive pressure ventilation. Transcutaneous oxygen saturation and PCO2, together with ECG were continuously monitored. Pulmonary artery blood flow velocity was measured non-invasively using pulsed wave Doppler. The 95% confidence intervals for the changes with and without CNEP in spontaneously breathing and ventilated patients showed no statistically significant changes in heart rate, O2 saturation, transcutaneous PCO2 or cardiac output. This study shows that the use of CNEP, administered in a tank respirator, does not lead to large changes in cardiac output.