The effect on measured DLco of different blood concentrations of COHb produced by breathing CO was studied in 19 healthy nonsmoking subjects. The DLco calculated without adjustment for blood COHb was found to be spuriously reduced by approximately 1% of the observed baseline value for each percent increase in blood COHb saturation. When measuring DLco of subjects who are likely to have elevated COHb blood levels, it is recommended that either blood COHb concentration or equilibrated alveolar PCO be determined in order to correct the DLco for the effect of COHb.