BACKGROUND: The heart rate variability (HRV)-derived Analgesia Nociception Index (ANI) is a continuous noninvasive tool to assess the nociception/antinociception balance in unconscious patients. It has been shown to be superior to haemodynamic variables in detecting insufficient antinociception in children, while little is known about its predictive value. METHODS: The primary objective of this prospective observational pilot study in paediatric surgical patients under sevoflurane anaesthesia was to compare the predictive value of the ANI and heart rate to help decide to give additional opioids. The paediatric anaesthesiologist in charge was blinded to ANI values. RESULTS: In patients with an ANI value <50 (indicating insufficient antinociception) at the moment of decision, ANI values dropped from +/-55 (indicating sufficient antinociception) to +/-35, starting 60 s before decision. Within 120 s after administration of fentanyl (1 mug/kg), ANI values returned to +/-60. This phenomenon was only observed in the ANI values derived from HRV data averaged over 2 min. Heart rate remained unchanged. In patients with ANI values >/=50 at the time of decision, opioid administration had no effect on ANI or heart rate. The same accounts for morphine for postoperative analgesia and fentanyl in case of intraoperative movement. CONCLUSIONS: This study provides evidence of a better predictive value of the ANI in detecting insufficient antinociception in paediatric surgical patients than heart rate. The same accounts for depicting re-establishment of sufficient antinociception after opioid drug administration. SIGNIFICANCE: In paediatric surgical patients anaesthetized with sevoflurane, the heart rate variability-derived Analgesia Nociception Index (ANI) appears to be a better predictor of insufficient antinociception than heart rate. The ANI also appears to depict re-establishment of sufficient antinociception better than heart rate.