BACKGROUND: Low-dose ketamine (0.25 - 0.5 mg/kg) has been shown to provide adequate analgesia for postoperative pain treatment. Ketamine at a subanesthetic dose blocks the delta-channel associated with the N-methyl-D-aspartate (NMDA) receptor, and this mechanism may underlie its analgesic effects. The aim of the present study was to dertermine if low-dose ketamine in addition to postoperative patient-controlled analgesia (PCA) potentiates the analgesic effect of opioids. METHODS: Sixty patients scheduled for total abdominal hysterectomy were divided into three groups. After awakening patients from general anesthesia, they received butorphanol 10 mg, ketorolac 240 mg and ondansteron 4 mg with normal saline 2 ml (control group) or ketamine 50 mg and normal saline 1 ml (ketamine 50 group) or ketamine 100 mg (ketamine 100 group) by PCA pump (1 ml of demand dose, 10 min lock-out interval and 100 ml total volume). The severity of pain, nausea, vomiting and sedation were assessed at 0, 1, 2, 6, 12, 24, 36 and 48 h after awakening. RESULTS: The total amount of butorphanol and ketorolac consumption was higher in the control group (8.6 mg, 206 mg) than in the ketamine 50 (6.2 mg, 148 mg) and ketamine 100 groups (5.3 mg, 127.2 mg). Ketamine consumption was higher in the ketamine 100 group (53.1 mg) than in the ketamine 50 group (31.7 mg). There was no difference among the groups in pain score, but the incidence of nausea and vomiting was higher in control group (40%) than in the other groups (10% each). Sedation scores were higher in the ketamine 100 group than in the other groups (P < 0.05). CONCLUSIONS: The addition of 50, 100 mg of ketamine decreases PCA drug consumption and incidence of nausea and vomiting (26 39%).