The current literature reports many measurements (arteriovenous oxygen content difference and cerebral metabolic rate of oxygen, etc.) with samples from the internal jugular veins (IJs), obtained from either side of the neck, based on the assumption that a reliable sample of mixed venous blood can be drawn. We compared oxygen saturation in both IJs in 32 patients with head injuries to establish the similarities or discrepancies in the two veins. Both IJs were cannulated with 20-G catheters; in five patients, a fiberoptic catheter was used to obtain a continuous recording of the hemoglobin saturation. Blood samples were taken simultaneously from the two IJs and immediately processed; the total number of samples processed was 342, with an average of 5.34 paired samples from each patient. The mean and the standard deviation of the differences between the saturation of the two IJs were, respectively, 5.32 and 5.15. Fifteen patients showed differences greater than 15% in hemoglobin saturation; three more patients showed differences greater than 10% at some point during the investigation. Ultimately, only eight patients had differences of less than 5%. No relationship was found among the computed tomographic scan data and the pattern of hemoglobin saturation detected. Therefore, we were not able to identify the side more appropriate for monitoring in patients with bilateral, predominantly monolateral, cortical, or deeply located lesions.(ABSTRACT TRUNCATED AT 250 WORDS)