Background and Study Aims: PCS is increasingly used in day-case surgery. Its use seems ideal for colonoscopy as an outpatient procedure. In theory PCS leads to faster recovery and earlier discharge. Patients and Methods: Between July 1999 and October 1999 patients undergoing colonoscopy as an outpatient procedure were randomized to receive intravenous injection of diazemuls 0.1mg/Kg and pethidine 0.5mg/Kg or a mixture of propofol 400mg and alfentanil 1 mg delivered via a Graseby 3300 PCA pump in bolus of 2mg/ml (lockout period of 3 minutes). Patients' pulse rate and oxygen saturation were monitored continuously and blood pressure taken every 5 minutes. Outcome parameters included procedure time, recovery time, pain score (un-scaled 10cm visual analogue score), desaturation (oxygen saturation < 90%) and hypotension (systolic blood pressure < 90mmHg). Results: 110 patients were recruited (55 in each group). The two groups were comparable in age, body weight and sex ratio. The mean procedure time was significantly longer in the PCS group (25 ± 13 vs. 17.6 ± 8 minutes). Desaturation occurred in 2 patients from the PCS group and 4 from the diazemul group (P=0.68). Fourteen from the PCS group and 16 from the diazemul group developed hypotension (P=0.67). The mean pain score was comparable in either group (5.4 vs. 4.6, P=0.49). A tendency towards shorter median recovery time was noted in the PCS group (20 vs. 30 mintues, P=0.08). Conclusions: PCS in outpatient colonoscopy led to slightly shorter recovery time at the expense of significantly longer procedure time.