A double-blind, randomized, placebo- and drug-controlled study in which the analgesic efficacy and safety of intravenous (i.v.) tramadol in patients with post-operative pain is reported. One hundred and eighty patients recovering from gynaecological or abdominal surgery were assigned to one of three treatment groups. After titration of an individual loading dose, patients could self-administer tramadol 20 mg, morphine 2 mg or placebo using a patient-controlled analgesia (PCA) device throughout a 48-h period. Criteria of efficacy were a decrease in pain intensity within the first 30 min of at least 20 on a visual analogue scale (VAS) (0 denotes no pain, 100 worst pain imaginable) and satisfactory analgesia in the patient's opinion during the study period. Patients treated with tramadol, morphine and placebo were assessed as responders at 66.7%, 75.0% and 18.3% (P < 0.0001). VAS after the initial bolus were 39.2 +/- 22.1, 35.9 +/- 21.6 and 50.0 +/- 24.2 (P = 0.002), the initial loading dose amounted to tramadol 144.9 +/- 51.2 mg, morphine 12.3 +/- 5.1 mg and placebo 17.2 +/- 4.9 mL. No serious opioid-related adverse events occurred in the patients given tramadol while two patients given morphine developed an impaired respiratory rate and a decreased oxygen saturation to 80% or less. Tramadol proved to be efficacious for PCA treatment of post-operative pain following gynaecological and abdominal surgery.