Background: Endogenous adenosine might cause or perpetuate bradyasystole. Our aim was to determine whether aminophylline, an adenosine antagonist, increases the rate of return of spontaneous circulation(ROSC) after out-of-hospital cardiac arrest. Methods: In a double-blind trial, we randomly assigned 971 patients older than 16 years with asystole or pulseless electrical activity at fewer than 60 beats per minute, and who were unresponsive to initial treatment with epinephrine and atropine, to receive intravenous aminophylline(250 mg, and an additional 250 mg if necessary)(n=486) or placebo(n=485). The patients were enrolled between January, 2001 and September, 2003, from 1886 people who had had cardiac arrests. Standard resuscitation measures were used for at least 10 mins after the study drug was administered. Analysis was by intention-to-treat. This trial is registered with the ClinicalTrials.gov registry with the number NCT00312273. Findings: Baseline characteristics and survival predictors were similar in both groups. The median time rom the arrival of the advanced life-support paramedic team to study drug administration was 13 min. The proportion of patients who had an ROSC was 24.5%in the aminophylline group and 23.7%in the placebo group(difference 0.8%; 95%CI-4.6%to 6.2%; p=0.778). The proportion of patients with non-sinus tachyarrhythmias after study drug administration was 34.6%in the aminophylline group and 26.2%in the placebo group(p=0.004). Survival to hospital admission and survival to hospital discharge were not significantly different between the groups. A multivariate logistic regression analysis showed no evidence of a significant subgroup or interactive effect from aminophylline. Interpretation: Although aminophylline increases non-sinus tachyarrhythmias, we noted no evidence that it significantly increases the proportion of patients who achieve ROSC after bradyasystolic cardiac arrest. Background: Endogenous adenosine might cause or perpetuate bradyasystole. Our aim was to determine whether aminophylline, an adenosine antagonist, increases the rate of return of spontaneous circulation (ROSC) after out-of-hospital cardiac arrest. Methods: In a double-blind trial , we randomly assigned 971 patients older than 16 years with asystole or pulseless electrical activity at fewer than 60 beats per minute, and who were unresponsive to initial treatment with epinephrine and atropine, to receive intravenous aminophylline (250 mg, and an additional 250 mg if necessary) (n = 486) or placebo (n = 485). The patient were enrolled between January, 2001 and September, 2003, from 1886 people who had had cardiac arrests. Standard resuscitation measures were used for at least 10 mins after the study drug was administered. Analysis was by intention-to-treat. This trial is registered with the Clinical Trials.gov registry with the NCT00312273. Findings: Baseline characteristics and survival pr The median time rom the arrival of the advanced life-support paramedic team to study drug administration was 13 min. The proportion of patients who had an ROSC was 24.5% in the aminophylline group and 23.7% in the placebo The proportion of patients with non-sinus tachyarrhythmias after study drug administration was 34.6% in the aminophylline group and 26.2% in the placebo group (p = 0.778) (difference 0.8%; 95% CI -4.6% to 6.2%; p = 0.778) = 0.004). Survival to hospital discharge and survival to hospital discharge were not significantly different between the groups. A multivariate logistic regression analysis showed no evidence of a significant subgroup or interactive effect from aminophylline. Interpretation: Although aminophylline increases non-sinus tachyarrhythmias, we noted no evidence that it significantly increases the proportion of patients who achieve ROSC after bradyasystolic cardiac arrest.