OBJECTIVE The electroencephalogram (EEG) is used in neonatal intensive care units to assess brain maturation and neurologic prognosis in preterm newborns. Most of these newborns are sedated by opioids because of long-term assisted ventilation. The aim of this study was to describe the effects of sufentanil on the EEG in preterm newborns and to evaluate the consequences of such a treatment on neurologic assessment. METHODS Fifteen preterm newborns <28 days of extrauterine life were studied. All of them were sedated by sufentanil (initial bolus injection of 0.5 micro g/kg, followed by continuous infusion of 0.2 micro g/kg/h). Three EEGs were performed: the first before and during the bolus injection, the second in the 48 hours after the start of the continuous infusion, and the third at least 24 hours after the treatment was stopped. Qualitative and quantitative methods were used to analyze each EEG. RESULTS EEG patterns were not affected by sufentanil treatment. Bolus injection and continuous infusion induced a significant increase of EEG discontinuity in preterm newborns affecting mean burst percentage and mean and maximum interburst duration. CONCLUSIONS The present data demonstrate that EEG is affected by bolus injection and continuous infusion of sufentanil. Sedation must therefore be considered to avoid misinterpretation of EEGs.