Background: Dexmedetomidine has advantages during colonoscopy as it allows the patient to cooperate during the procedure. Few studies examined the dexmedetomidine-remifentanil combination. This study was to evaluate the effects of different doses of the dexmedetomidine-remifentanil combination in colonoscopy. Methods: This was a prospective trial carried out at the Fourth Hospital of Hebei Medical University between 02/2018 and 10/2018. The patients were randomized: group I (dexmedetomidine 0.2 mu g.kg(-1)), group II (dexmedetomidine 0.3 mu g.kg(-1)), and group III (dexmedetomidine 0.4 mu g.kg(-1)), all combined with remifentanil. The primary outcomes were the patient's body movements during the procedure and adverse events. Results: Compared with at admission (T-0), the SBP, HR, and RR at immediately after giving DEX (T-1), at the beginning of the examination (T-2), 5 min after the beginning of the examination (T-3), 10 min after the beginning of the examination (T-4), and at the end of the examination (T-5) in the three groups were all reduced (all P < 0.05), but all were within the clinically normal range. SpO(2) remained > 98% in all patients during the examination. Compared with T-0, the BIS values of the three groups were decreased at T-1 and T-2 (allP < 0.05). There were no significant differences in BIS among the three groups (allP > 0.05). The minimum BIS value in group III was lower than in groups I and II (P < 0.05). The degree of satisfaction with the anesthesia effect was higher in groups II and III that in group I (P < 0.05). No hypotension occurred, seven patients had bradycardia, and four patients had nausea/vomiting. Conclusions: Dexmedetomidine 0.3 mu g.kg(-1) combined with remifentanil was effective for colonoscopy and had few adverse reactions.