Prospective assessment of the accuracy of three pulse oximeters and two probe sites in darkly pigmented critically ill patients under clinical conditions. One hundred consecutive, darkly pigmented critically ill adult patients with arterial lines in situ were studied. Patients were excluded if the haemoglobin concentration was less than 7 g/dl and carboxyhaemoglobin or methaemoglobin levels exceeded 2%. Pigmentation was objectively quantified with a portable EEL reflectance spectrophotometer (Evans Electroselenium Company, Diffusion Systems Limited, London). Reflectance was measured at nine wavelengths. The degree of pigmentation as measured by percentage reflectance closely matched that of a control group of black Africans from a pigmentation study. The limits of agreement (2.6% to 5.8%), precision and bias values between pulse oximeter and co-oximeter readings fell within a narrow range. The 95% confidence intervals of the limits of agreement reflected a small variation in the difference between pulse oximeter and co-oximeter readings. These small differences were not clinically significant in the pigmented patients who were enrolled in the study. The accuracy of pulse oximetry is not adversely affected by skin pigmentation, and it remains a useful oxygenation monitoring device in darkly pigmented patients.