OBJECTIVE Colonoscopy is a painful procedure. Therefore conscious sedation is often used. However, the value of adding analgesics to sedatives has not been well evaluated. METHODS The double blind, randomized controlled trial was carried out to compare patients' and endoscopist' assessments in both groups of patients: MP (Midazolam/Placebo) group (n=49) received midazolam plus placebo and MM (Midazolam/Meperidine) group (n=51) received midazolam plus meperidine. RESULTS There was no significant difference of baseline characteristics except previous operation history. There were no significant difference of grade of tolerance, pain and willingness to another colonoscopy between the two groups. In endoscopist' satisfaction, the degree of difficulty was higher in MP group than in MM group (39.0 vs. 31.7,p<0.05). After the colonoscopy, systolic blood pressure, oxygen saturation, and pulse rate were significantly decreased (p<0.05) in both groups. However, there was no difference in the degree of decrease between the two groups. The incidence of adverse effect was not different in the two groups. However, one case of orthostatic hypotension with presyncope was noted in MM group. CONCLUSIONS Adding meperidine to the midazolam before the colonoscopy does not seem to bring more beneficial effect to patients, whereas endoscopist favored the use of both medications.