Pulmonary enzyme heme oxygenase, which catalyses carbon monoxide production, may be responsible for arteriovenous carboxyhemoglobin (COHb) differences measured in humans. Unspecific inflammatory stimuli have been shown to induce pulmonary heme oxygenase possibly leading to increased pulmonary carbon monoxide production and elevated arterial COHb. Arteriovenous COHb gradients may therefore be a measurable parameter of lung injury severity. To exclude a technical artefact, we repeated measurements of central venous COHb and arterial COHb in healthy humans (ASA I-II) undergoing elective surgery with the ABL 625 and the updated version, ABL 725 (Radiometer, Copenhagen). In addition to the standard calibration, an especially accurate adjustment of the spectrophotometer wavelengths (SAT100) was performed. This adjustment eliminates the FCOHb dependency on the oxygen saturation. No significant differences were detectable between central venous and arterial COHb concentrations with either blood gas analyzer. The difference between central venous COHb and arterial COHb was 0.09 with the ABL 625 and -0.03 with the ABL 725. Therefore, we conclude that previously reported arteriovenous COHb differences are artifactual and may be eliminated by SAT 100 adjustment, as is possible with the ABL 725.