The reproducibility of the diffusion capacity of the lungs during steady state at rest using end-tidal determination of carbon monoxide was studied during repeated examinations of DLCO in 136 patients with diverse pulmonary diseases. The correlation coefficients between these sets of values for determination of carbon monoxide in the inspired, expired gas and at the end of the tidal volume were high (0.996–0.963). Thus the correlation coefficients for extraction of CO for the DLCOss2 were excellent (0.951 and 0.932 respectively). This indicates that DLCOss2 is highly reproducibl