The course of arterial to end-tidal carbon dioxide tension difference [P(a-ET) co2] was evaluated during general anesthesia in 25 patients scheduled for renal surgery performed in the “kidney position.” The difference between arterial Pco2 (Paco2) corrected to body temperature, and end-tidal Pco2 (PETCO2) measured by mass spectrometry was assessed after induction of anesthesia, after placement in the lateral decubitus position with back arched over a kidney bridge (“kidney position”), and every 20 min until the patients were replaced in the supine position at the end of the surgical procedure. Heart rate, arterial blood pressure, and esophageal temperature were simultaneously recorded. After induction of anesthesia, when the patients were lying supine (Tl), P(a-ET) co2 was 4.8 ± 3.9 mm Hg (mean ± SD). Placing the patients in the kidney position (T2) induced a significant increase in P(a-ET) co2 (to 7.9 ± 3.5 mm Hg; P < 0.01). these alterations occurred without any significant change in mean arterial blood pressure or heart rate. a progressive increase in mean p(a-et) co2 occurred with maintenance of anesthesia; p(a-et) co2 reached 8.8 ±4.1 mm hg (p < 0.05 vs t2) and 8.9 ± 4.4 mm hg (p < 0.05 vs t2) at 65 and 85 min, respectively, after lateral decubitus positioning. large variations between and within patients were observed. although stable mean arterial pressure was maintained, these changes were associated with a significant decrease in body temperature. these results demonstrate that p(a-et) co2 increases when patients are placed in the kidney position and may vary with the prolongation of anesthesia in this situation. thus, PETCO2 can be affected by position, and thus may not be constantly of value when estimating Paco2 during anesthesia.