Objective: Investigate the temporal development of EEG and prognosis. Methods: Prospective observational substudy of the Target Temperature Management trial. Six sites performed simplified continuous EEG-monitoring (cEEG) on comatose patients after cardiac arrest, blinded to treating physicians. We determined time-points of recovery of a normal-voltage continuous background activity and the appearance of an epileptiform EEG, defined as abundant epileptiform discharges, periodic/rhythmic discharges or electrographic seizure activity. Results: 134 patients were included, 65 had a good outcome. Early recovery of continuous background activity (within 24 h) occurred in 72 patients and predicted good outcome since 55 (76%) had good outcome, increasing the odds for a good outcome seven times compared to a late background recovery. Early appearance of an epileptiform EEG occurred in 38 patients and 34 (89%) had a poor outcome, increasing the odds for a poor outcome six times compared to a late debut. The time to background recovery and the time to epileptiform activity were highly associated with outcome and levels of neuron-specific enolase. Multiple regression analysis showed that both variables were independent predictors. Conclusions: Time to epileptiform activity and background recovery are independent prognostic indicators. Significance: Patients with early background recovery combined with late appearance of epileptiform activity may have a good outcome. (C) 2018 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.