BACKGROUND Many anesthetic drugs have been shown to disrupt conscious recall (explicit memory) in volunteers. However, unconscious processing (implicit memory) of intraoperative auditory material may occur during general anesthesia and may provide an opportunity for intraoperative therapeutic intervention. In this study, we examined patients undergoing elective cardiac surgery for evidence of intraoperative implicit and explicit memory. RESULTS Twenty-five subjects provided written informed consent and underwent general anesthesia and cardiopulmonary bypass for cardiac surgery. During the operation, patients were randomized to receive 1 of 2 different audiotapes of associated word pairs. Postoperatively, a blinded observer conducted a standardized interview to determine the extent of intraoperative implicit and explicit memory. With the use of free association, significant intraoperative implicit memory was found. In contrast, no patient had spontaneous or directed recall of intraoperative events, and we did not find evidence of intraoperative explicit memory with a recognition task. CONCLUSIONS Patients undergoing general anesthesia for cardiac surgery were reliably able to reinforce associations between word pairs solely on the basis of their intraoperative presentation. This provides further evidence that patients are capable of processing intraoperative auditory information.