IMPORTANCE Out-of-hospital cardiac arrest (OHCA) has poor outcome. Whether intra-arrest transport, extracorporeal cardiopulmonary resuscitation (ECPR), and immediate invasive assessment and treatment (invasive strategy) is beneficial in this setting remains uncertain. OBJECTIVE To determine whether an early invasive approach in adults with refractory OHCA improves neurologically favorable survival. DESIGN, SETTING, AND PARTICIPANTS Single-center, randomized clinical trial in Prague, Czech Republic, of adults with a witnessed OHCA of presumed cardiac origin without return of spontaneous circulation. A total of 256 participants, of a planned sample size of 285, were enrolled between March 2013 and October 2020. Patients were observed until death or day 180 (last patient follow-up ended on March 30, 2021). INTERVENTIONS In the invasive strategy group (n = 124), mechanical compression was initiated, followed by intra-arrest transport to a cardiac center for ECPR and immediate invasive assessment and treatment. Regular advanced cardiac life support was continued on-site in the standard strategy group (n = 132). MAIN OUTCOMES AND MEASURES The primary outcome was survival with a good neurologic outcome (defined as Cerebral Performance Category [CPC] 1-2) at 180 days after randomization. Secondary outcomes included neurologic recovery at 30 days (defined as CPC 1-2 at any time within the first 30 days) and cardiac recovery at 30 days (defined as no need for pharmacological or mechanical cardiac support for at least 24 hours). RESULTS The trial was stopped at the recommendation of the data and safety monitoring board when prespecified criteria for futility were met. Among 256 patients (median age, 58 years; 44 [17%] women), 256 (100%) completed the trial. In the main analysis, 39 patients (31.5%) in the invasive strategy group and 29 (22.0%) in the standard strategy group survived to 180 days with good neurologic outcome (odds ratio [OR], 1.63 [95% CI, 0.93 to 2.85]; difference, 9.5% [95% CI, -1.3% to 20.1%]; P = .09). At 30 days, neurologic recovery had occurred in 38 patients (30.6%) in the invasive strategy group and in 24 (18.2%) in the standard strategy group (OR, 1.99 [95% CI, 1.11 to 3.57]; difference, 12.4% [95% CI, 1.9% to 22.7%]; P = .02), and cardiac recovery had occurred in 54 (43.5%) and 45 (34.1%) patients, respectively (OR, 1.49 [95% CI, 0.91 to 2.47]; difference, 9.4% [95% CI, -2.5% to 21%]; P = .12). Bleeding occurred more frequently in the invasive strategy vs standard strategy group (31% vs 15%, respectively). CONCLUSIONS AND RELEVANCE Among patients with refractory out-of-hospital cardiac arrest, the bundle of early intra-arrest transport, ECPR, and invasive assessment and treatment did not significantly improve survival with neurologically favorable outcome at 180 days compared with standard resuscitation. However, the trial was possibly underpowered to detect a clinically relevant difference.