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Aim: The aim of this prospective, randomized, double-blind study was to evaluate whether adding metamizol to tramadol in an i.v. patient controlled analgesia (PCA) system improved the analgesic efficacy compared to i.v. PCA with tramadol plus metamizol administered when needed. Methods: Forty-six adult patients undergoing elective hemicolectomy were randomly assigned to either a tramadol group (group T; n = 23) or a tramadol plus metamizol group (group TM; n = 23) for postoperative pain treatment. Loading doses of i.v. 100 mg tramadol (group T) or i.v. 100 mg tramadol plus 1000 mg metamizol (group TM) were administered intra-operatively. Postoperatively, the PCA pump was programmed to deliver, on demand, doses of tramadol 20 mg (group T) or tramadol 20 mg plus metamizol 200 mg (group TM) without any baseline infusion. Lock-out time was set to 5 min and the 4-h dose limit was 400 mg and 4000 mg for tramadol and metamizol, respectively. Group T patients received i.v. metamizol when necessary as a rescue medication. Pain intensity (NRS), patient satisfaction, tramadol consumption and side effects were the outcome measures. Results: The first effective pain relief was reached at 4 h in group TM patients (NRS in group TM, 3.22; in group T, 4.57; P < 0.001); however, beyond 4 h. pain relief was similar. The incidence and severity score of postoperative nausea and vomiting were low in the TM group (P < 0.05). There were no significant differences between groups in terms of patient satisfaction. Conclusions: Tramadol combined with metamizol in a concentration of 1:10 mg in the PCA system provided slightly earlier pain relief and a decrease in the nausea/vomiting score compared to tramadol PCA plus, when needed, metamizol. Adding metamizol to a PCA system may be a practical and convenient option for postoperative analgesia. [ABSTRACT FROM AUTHOR]