It is often neglected that hypercapnia may occur during upper epidural anesthesia. To evaluate the necessity and accuracy of P_(ET)CO_2 monitoring through a nasotracheal oropharyngeal cathter during upper epidural anesthesia, 16 patients undergoing radical excision of mammary carcinoma, were investigated for the changes of PaCO_2 and PET CO_2, and the correlation between them. The results showed that the increases of PaCO_2 during the anesthesia were significant (P