BACKGROUND: Clonidine has been known to be an alpha2-agonist and may potentiate the anesthetic effect. Clonidine also suppresses hemodynamics, which confounds the indices of anesthetic depth. Thus, the author regarded patient's movement as a standard of anesthetic depth as well as hemodynamic change. Interaction between clonidine and propofol in clinical anesthesia was evaluated when given in clinical dose. METHODS: Forty patients scheduled for spine surgery were randomly assigned into two groups; propofol only (control group, n = 20) and propofol with clonidine (study group, n = 20). In the study group, clonidine was administered intravenously. Anesthesia was inducted with propofol TCI and N2O, and then the effect site concentration of propofol was controlled. The changes of mean arterial pressure, heart rate, and the effect site concentration was observed and analyzed at baseline, before and after skin incision, and then EC50 was obtained and analyzed. RESULTS: Mean arterial pressure and heart rate significantly changed between before and after the skin incision in both groups. There was no significant difference of propofol EC50 in the prevention of patient's movement at the time of skin incision. CONCLUSIONS: The authors concluded that preoperative administration of clinical dose clonidine provides stable hemodynamic conditions in propofol-N2O anesthesia, but no changes in the depth of anesthesia.