Objective: To describe characteristics of patients admitted with refractory cardiac arrest for possible extracorporeal cardiopulmonary resuscitation (ECPR) and gain insight into the reasons for refraining from treatment in some. Methods: Nationwide retrospective cohort study involving all tertiary centres providing ECPR in Denmark. Consecutive patients admitted with ongoing chest compression for evaluation for ECPR treatment were enrolled. Presenting characteristics, duration of no-flow and low-flow time, end-tidal carbon dioxide (ETCO2), lactate and pH, and recording of reasons for refraining from ECPR documented by the treating team were recorded. Outcomes were survival to intensive care unit admission and survival to hospital discharge. Results: Of 579 patients admitted with refractory cardiac arrest for possible ECPR, 221 patients (38%) proceeded to ECPR and 358 patients (62%) were not considered candidates. Median prehospital low-flow time was 70 min (IQR 56 to 85) in ECPR patients and 62 min (48 to 81) in no-ECPR patients, p<0.001. Intra-arrest transport was more than 50 km in 92 (42%) ECPR patients and 135 in no-ECPR patients (38%), p=0.25. The leading causes for not initiating ECPR stated by the treating team were duration of low-flow time in 39%, severe metabolic derangement in 35%, and in 31% low ETCO2. The prevailing combination of contributing factors were non-shockable rhythm, low ETCO2, and metabolic derangement or prehospital low-flow time combined with low ETCO2. Survival to discharge was only achieved in six patients (1.7%) in the no-ECPR group. Conclusions: In this large nationwide study of patients admitted for possible ECPR, two-thirds of patients were not treated with ECPR. The most frequent reasons to abstain from ECPR were long duration of prehospital low-flow time, metabolic derangement and low ETCO2.