BACKGROUND: There is a high incidence of pain associated with an intravenous injection of propofol, and different methods have been used in an attempt to minimize the incidence and severity of this pain. This study compared the effect of a ketamine pretreatment with that of a remifentanil pretreatment on the pain associated with a propofol injection. METHODS: 225 ASA physical status 1-2 adult patients scheduled for elective surgery were enrolled into this randomized double blind study. The patients received 2 ml of saline (n = 45), 2 ml of ketamine 20 mg (n = 45), 2 ml of remifentanil 0.01 mg (n = 45), or 2 ml of remifentanil 0.02 mg (n = 45) or 2 ml of remifentanil 0.03 mg (n = 45) 30 seconds prior to administering 2 mg/kg of 1% propofol. An anesthesiologist who was blinded to the study asked the patients to evaluate the pain using a four-point scale (none, mild, moderate, severe). RESULTS: The remifentanil 0.02 mg, remifentanil 0.03 mg and ketamine groups showed significantly less frequent and intense pain than the saline group (P < 0.05). The remifentanil 0.02 mg and 0.03 mg provided as much pain relief as ketamine (P > 0.05), but the remifentanil 0.01 mg did not (P < 0.05). There was a similar incidence of injection pain in the remifentanil 0.02 mg and remifentanil 0.03 mg groups. CONCLUSIONS: An intravenous remifentanil 0.02 mg or remifentanil 0.03 mg pretreatment is as effective in alleviating the pain associated with a propofol injection as a intravenous ketamine pretreatment. The remifentanil 0.03 mg pretreatment had a similar analgesic effect relative to the remifentanil 0.02 mg pretreatment.