Objective: To assess the value of electroencephalogram for prediction of outcome of comatose patients after cardiac arrest treated with mild therapeutic hypothermia. Design: Prospective cohort study. Setting: Medical ICU. Patients: One hundred forty-two patients with postanoxic encephalopathy after cardiac arrest, who were treated with mild therapeutic hypothermia. Measurements and Main Results: Continuous electroencephalogram was recorded during the first 5 days of ICU admission. Visual classification of electroencephalogram patterns was performed in 5-minute epochs at 12 and 24 hours after cardiac arrest by two independent observers, blinded for patients’ conditions and outcomes. Patterns were classified as isoelectric, low voltage, epileptiform, burst-suppression, diffusely slowed, or normal. Burst-suppression was subdivided into patterns with and without identical bursts. Primary outcome measure was the neurologic outcome based on each patient’s best achieved Cerebral Performance Category score within 6 months after inclusion. 67 patients (47%) had favorable outcome (Cerebral Performance Category, 1–2). In patients with favorable outcome, electroencephalogram patterns improved within 24 hours after cardiac arrest, mostly toward diffusely slowed or normal. At 24 hours after cardiac arrest, the combined group of isoelectric, low voltage, and “burst-suppression with identical bursts” was associated with poor outcome with a sensitivity of 48% (95% CI, 35–61) and a specificity of 100% (95% CI, 94–100). At 12 hours, normal or diffusely slowed electroencephalogram patterns were associated with good outcome with a sensitivity of 56% (95% CI, 41–70) and a specificity of 96% (95% CI, 86–100). Conclusions: Electroencephalogram allows reliable prediction of both good and poor neurologic outcome of patients with postanoxic encephalopathy treated with mild therapeutic hypothermia within 24 hours after cardiac arrest.