Recording of electroencephalogram (EEG) is of value to estimate vigilance states in children as in adults. In order to determine the diagnostic and prognostic value of emergency EEG in case of mental confusion, behavioral disorders and vigilance disorders in childhood, we conducted a retrospective study in 397 children (aged 2 months to 16 years). EEG was recorded less than 24 hours after an emergency consultation for acute confusion or acute behavioral disorder (n = 106) or after admission to the intensive care unit for comatose stage (n = 291). EEG gave diagnostic indications mainly in case of convulsive pathology or hepatic encephalopathy. In comatose children, we established a 4-stage EEG scale of increasing severity. This classification was compared to EEG scales already published in the literature and appeared very similar to that from Pampiglione and Harden, established in 150 children after cardiac arrest. A highly poor prognostic value was associated with burst-suppression post-anoxic patterns and with isoelectric records signaling brain death. Our classification of emergency EEG patterns is mainly helpful in these two situations, but does not exclude strict and repeated clinical and EEG follow-up in other cases, as a relatively preserved initial EEG may later deteriorate.