Oximetric measurements are influenced by several mechanisms. Severe jaundice is one of these mechanisms with some clinical interest. In the literature it is pointed out that a high bilirubin concentration may falsify oximetric measurements and is often accompanied by elevated COHb levels. The reason for this phenomenon is thought to be an interference in the absorption spectra of haemoglobin derivatives and bilirubin [2, 3, 4, 10]. In our investigation we attempted to answer the following questions: 1. How do multiwavelength oximeters measure haemoglobin derivatives in different bilirubin concentrations? 2. Do different multiwavelength oximeters give different concentrations of haemoglobin derivatives? METHODS. In 13 patients who developed postoperative jaundice on the intensive care unit, O2Hb, COHb and MetHb were measured in mixed venous blood with two multiwavelength oximeters (OSM3, Radiometer; CO 2500, Ciba-Corning). Bilirubin concentration was measured by the DPD (dichlorphenyldiazonium) method in the central laboratory of our hospital. RESULTS. With increasing bilirubin concentrations, both oximeters measured increasing O2Hb values; the OSM3 consistently showed higher O2Hb concentrations than the CO 2500, with a maximal difference of 2.8% (Fig. 3). Regarding COHb, we saw clear increases in the values with increasing bilirubin concentrations (Fig. 4). The CO 2500 showed higher COHb values than the OSM3 (average 1.54 +/- 0.3%). The findings regarding MetHb differed. The CO 2500 showed increasing MetHb values as the bilirubin concentration increased (Fig. 5). All measurements exceeded normal values above a bilirubin concentration of 17 mg/dl. The OSM3, however, measured constant MetHb values which did not depend on jaundice. CONCLUSIONS. 1. The in vitro measurement of haemoglobin derivates by multiwavelength oximeters is influenced by hyperbilirubinaemia. This is caused by an interference between the light absorption spectra of the haemoglobin derivates and of bilirubin and by the increasing development of endogenous CO in the haem metabolism during severe jaundice (Fig. 7). 2. With increasing bilirubin levels, a lower O2Hb is measured with the CO 2500 than with the OSM3. 3. We also see increasing COHb values with rising bilirubin concentrations. 4. With increasing bilirubin levels the MetHb concentration measured with the CO 2500 rises, while the OSM3 gives constant MetHb values. 5. In severe jaundice the O2Hb values measured with multiwavelength oximeters are not identical with the real blood concentration of this haemoglobin derivative. In this situation multiwavelength oximeters cannot be used as a reference method for in vivo oximetric systems such as pulse oximeters or fibreoptic pulmonary artery catheters.