IMPORTANCE: Experimental evidence suggests that cyclosporine prevents postcardiac arrest syndrome by attenuating the systemic ischemia reperfusion response. OBJECTIVE: To determine whether early administration of cyclosporine at the time of resuscitation in patients with out-of-hospital cardiac arrest (OHCA) would prevent multiple organ failure. DESIGN, SETTING, AND PARTICIPANTS: A multicenter, single-blind, randomized clinical trial was conducted from June 22, 2010, to March 13, 2013 (Cyclosporine A in Out-of-Hospital Cardiac Arrest Resuscitation [CYRUS]). Sixteen intensive care units in 7 university-affiliated hospitals and 9 general hospitals in France participated. A total of 6758 patients who experienced nonshockable OHCA (ie, asystole or pulseless electrical activity) were assessed for eligibility. Analyses were performed according to the intention-to-treat analysis. INTERVENTIONS: Patients received an intravenous bolus injection of cyclosporine, 2.5 mg/kg, at the onset of advanced cardiovascular life support (cyclosporine group) or no additional intervention (control group). MAIN OUTCOMES AND MEASURES: The primary end point was the Sequential Organ Failure Assessment (SOFA) score, assessed 24 hours after hospital admission, which ranges from 0 to 24 (with higher scores indicating more severe organ failure). Secondary end points included survival at 24 hours, hospital discharge, and favorable neurologic outcome at discharge. RESULTS: Of the 6758 patients screened, 794 were included in intention-to-treat analysis (cyclosporine, 400; control, 394). The median (interquartile range [IQR]) ages were 63.0 (54.0-71.8) years for the cyclosporine group and 66.0 (57.0-74.0) years for the control group. The cohorts included 293 men (73.3%) in the treatment group and 288 men (73.1%) in the control group. At 24 hours after hospital admission, the SOFA score was not significantly different between the cyclosporine (median, 10.0; IQR, 7.0-13.0) and the control (median, 11.0; IQR, 7.0-15.0) groups. Survival was not significantly different between the 98 (24.5%) cyclosporine vs 101 (25.6%) control patients at hospital admission (adjusted odds ratio [aOR], 0.94; 95% CI, 0.66-1.34), at 24 hours for 67 (16.8%) vs 62 (15.7%) patients (aOR, 1.08; 95% CI, 0.71-1.63), and at hospital discharge for 10 (2.5%) vs 5 (1.3%) patients (aOR, 2.00; 95% CI, 0.61-6.52). Favorable neurologic outcome at discharge was comparable between the cyclosporine and control groups: 7 (1.8%) vs 5 (1.3%) patients (aOR, 1.39; 95% CI, 0.39-4.91). CONCLUSION AND RELEVANCE: In patients presenting with nonshockable cardiac rhythm after OHCA, cyclosporine does not prevent early multiple organ failure. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT01595958; EudraCT Identifier: 2009-015725-37.