BACKGROUND: When polysomnography is indicated in a patient with a presumed sleep disorder, continuous monitoring of arterial carbon dioxide tension (PaCO2) is desirable, especially if nocturnal hypoventilation is suspected. Transcutaneous CO2 monitors (PtcCO2) provide a noninvasive correlate of PaCO 2, but their accuracy and stability over extended monitoring have been considered inadequate for the diagnosis of hypoventilation. We examined the stability and accuracy of PtcCO measurements and the performance of a previously described linear interpolation technique designed to correct for calibration drift. METHODS: We compared the PtcCO values from 2 TINA TCM-3 monitors to PaCO values from arterial blood samples obtained at the beginning, every 15 min of the first hour, and then hourly over 8 hours of monitoring in 6 hemodynamically stable, male, intensive care patients (mean age 46 ± 17 y). RESULTS: Time had a significant (P =.002) linear effect on the PtcCO -PaCO difference, suggesting calibration drift over the monitoring period. We found no differences between monitor type or interaction between time and monitor type. For the 2 monitors the uncorrected bias was 3.6 mm Hg and the limits of agreement were -5.1 to 12.3 mm Hg. Our linear interpolation algorithm improved the bias and limits of agreement to 0.4 and -5.5 to 6.4 mm Hg, respectively. CONCLUSIONS: Following stabilization and correction for both offset and drift, PtcCO tracks PaCO with minimal residual bias over 8 hours of monitoring. Should future research confirm these findings, then interpolated PtcCO may have an increased role in detecting sleep hypoventilation and assessing the efficacy of treatment.