Since traditional pulmonary function testing is centered on measurements of air flow and lung volume, a method to assess the pulmonary circulation might improve our ability to evaluate diseases that impact upon pulmonary hemodynamics. We have developed a PC based application that rapidly calculates pulmonary blood flow. Subjects rebreath a mixture of 10% argon and 3.5% freon for 20 seconds. Gas concentrations at the mouth are monitored by a clinical mass spectrometer and signals are acquired and processed with off-the-shelf hardware. To test the accuracy and reproducibility of this technique, patients with pulmonary artery catheters were assessed by standard thermodilution methods and the rebreathing test. Measurements using this non-invasive technology closely corelate with invasive thermodilution methods (r = 0.980) and show equivalent reproducibility (average standard error = 2.5%). This application of signal processing technology can extend the role of pulmonary function testing to include routine evaluation of the pulmonary circulation.