The relationship between the pulmonary excretion rate of carbon monoxide (VECO) and the concentration of CO, in a sample of breath, drawn through a nasopharyngeal catheter at end-expiration, was assessed in 25 studies of nine preterm and 14 term infants. The VECOand this approximate end-tidal sample of CO (ETCO) correlated significantly over a wide range of CO elimination rates: VECO= 10.45 ETCO+ 2.25 (n = 25, r = 0.95). The ETCOcorrectly predicted elevations in VECO> 2 SD of the mean VECOfor normal infants (13.9 ± 3.5 μl/kg/h), with 90% sensitivity and 73% specificity (p < 0.01). Three subjects with Rh isoimmune hemolytic disease were easily identified by the ETCOas well as the VECO. The ETco is a simple, noninvasive measurement for rapidly identifying infants with significant hemolytic disease.