Abstract. Objective: To assess the validity of early sensory evoked potential (SEP) recording for reliable outcome prediction in comatose cardiac arrest survivors within 48h after restoration of spontaneous circulation (ROSC). Design and setting: Prospective cohort study in a medical intensive care unit of a university hospital. Patients: Twenty-five comatose, mechanically ventilated patients following cardiopulmonary resuscitation Measurements and results: Median nerve short- and long-latency SEP were recorded 4, 12, 24, and 48h after ROSC. Cortical N20 peak latency and cervicomedullary conduction time decreased (improved) significantly between 4, 12, and 24h after resuscitation in 22 of the enrolled patients. There was no further change in short-latency SEP at 48h. The cortical N70 peak was initially detectable in seven patients. The number of patients with increased N70 peak increased to 11 at 12h and 14 at 24h; there was no further change at 48h. Specificity of the N70 peak latency (critical cutoff 130ms) increased from 0.43 at 4h to 1.0 at 24h after ROSC. Sensitivity decreased from 1.0 at 4h to 0.83 at 24h after ROSC. Conclusion: Within 24h after ROSC there was a significant improvement in SEP. Therefore we recommend allowing a period of at least 24h after cardiopulmonary resuscitation for obtaining a reliable prognosis based on SEP.