SummaryThis article reviews the available literature on the validity and reliability of the non-invasive techniques, commonly known as CO2rebreathing, for estimating cardiac output. The differing indirect methodologies are described and illustrated.A table, constructed from the available literature, comparing criterionversusestimated cardiac outputs is presented. The varying combinations of methods employed, differing measurement conditions, i.e. rest and exercise, and divergent populations are illustrated and discussed. The correlation between criterion and estimated cardiac output for these studies ranged from r = 0.09 to 0.96, with a % standard deviation of the differences of 1.5 to 176.8%.The Collier and end-tidal methods, in conjunction with either the Comroe or McHardy CO2dissociation curve appears to be the most established, valid and reliable combination of methods for estimating resting cardiac output. These methods appear to be comparable to the combination of the Defares, end-tidal and Comroe curve methods for estimating cardiac output during exercise.Because of the potential for large errors, caution is urged when interpreting cardiac output results based on indirect estimation for individual assessment, or for subjects with certain types of pulmonary or heart diseases.