OBJECTIVE: Several reports have shown the negligible adverse effects of low-dose remifentanil on the autonomic nervous system. We propose that the administration of low-dose remifentanil would be beneficial without adverse respiratory and hemodynamic effects. This study aimed to examine the effects of low-dose remifentanil on postoperative pain relief and heart rate variability (HRV) after surgery. METHODS: In total, 20 patients, who underwent breast cancer surgery, were analysed for HRV in the post-anaesthesia care unit (PACU). A sedative dose of remifentanil was continuously infused if patients experienced pain while in PACU. The remifentanil infusion dose was determined by achieving analgesia with no adverse effects on hemodynamics and/or respiration. Variables of low-frequency power, high-frequency power and low-frequency power/high-frequency power ratio were measured before and after the administration of remifentanil. Pain score was expressed as the numeric rating scale (NRS) from 0 to 10. RESULTS: The mean dosage of remifentanil administered as a continuous infusion was 0.029+/-0.0042 mug kg(-1) min(-1). After remifentanil administration, the value of the NRS decreased from 4.2+/-2.9 to 2.7+/-2.6. In addition, the value of high-frequency power increased from 35.6+/-14.3 to 49.4+/-3.0. CONCLUSION: The continuous infusion of low-dose remifentanil may reduce post-operative pain scores and trigger the relative activation of the parasympathetic nervous system in post-surgical patients. This indicates that continuous infusion of low-dose remifentanil may be a useful option for postoperative pain relief in cases where postoperative pain control proves inadequate even with the application of regional block technique.