Abstract Study objective To evaluate whether the use of remifentanil to supplement propofol during spontaneous respiration confers any benefits in terms of quality of sedation and recovery, or in terms of reduction in propofol requirements. Design Prospective, randomized, double-blind study. Setting University hospital. Patients 50 ambulatory adult ASA physical status I and II patients scheduled for total colonoscopy. Interventions Patients were randomized to receive either propofol alone or propofol plus remifentanil 0.1 μg/kg/min, while independently maintaining spontaneous respiration. Measurements Cardiovascular and respiratory variables were measured before induction and at 1-minute intervals thereafter. Recovery from anesthesia was assessed using simple verbal commands and the Steward Post Recovery Score. Patient satisfaction was measured with a visual analog scale. Computer simulation was used to calculate the effect-site concentrations of propofol and remifentanil. Main results The depressant effects on blood pressure and respiratory function were significantly higher when propofol and remifentanil were combined. Although the addition of remifentanil resulted in a decrease of propofol usage, recovery of anesthesia was faster and patient satisfaction was higher when using propofol alone. Conclusions The addition of remifentanil to propofol during spontaneous ventilation offered no benefits compared with the use of propofol alone.