OBJECTIVE We conducted an open, prospective, randomized study to compare the efficacy, safety and recovery characteristics of remifentanil or propofol during monitored anaesthesia care in patients undergoing colonoscopy. METHODS Forty patients were randomly assigned to receive either propofol (1 mg kg(-1) followed by 10 mg kg (-1) h(-1), n = 20) or remifentanil (0.5 microg kg(-1) followed by 0.2 microg kg(-1) min(-1), n = 20). The infusion rate was subsequently adapted to clinical needs. RESULTS In the propofol group, arterial pressure and heart rate decreased significantly from the baseline. These variables remained unchanged in the remifentanil group, but hypoventilation occurred in 55% of patients. Early recovery was delayed in the propofol group (P < 0.002). Recovery of cognitive and psychomotor functions was faster in the remifentanil group. Fifteen minutes after anaesthesia, the Digit Symbol Substitution Test score was 28.6 +/- 12.8 versus 36.2 +/- 9.4 and the Trieger Dot Test score was 25.6 +/- 8.1 versus 18.7 +/- 4.1 in the propofol and remifentanil groups, respectively (both P < 0.05). Patient satisfaction, using a visual analogue scale, was higher in the propofol group (96 +/- 7 versus 77 +/- 21, P < 0.001). CONCLUSIONS Remifentanil proved efficient in reducing pain during colonoscopy. Emergence times were shorter and the recovery of cognitive function was faster with remifentanil compared with propofol. Remifentanil provided a smoother haemodynamic profile than propofol; however, the frequent occurrence of remifentanil-induced hypoventilation requires the cautious administration of this agent.