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Background: Adenosine test was proposed as a tool for identification of syncopal patients who benefit from pacemaker implantation. Aim of the study was to assess the relationship between adenosine levels, the outcome of adenosine test and results of implantable loop recorder (ILR) monitoring in patients with syncope., Methods: In 29 patients (mean age 59 ± 11 years, 15 men, 14 women) with unexplained syncope ILR was implanted. In addition, adenosine test (intravenous injection of 20 mg adenosine bolus) and assays of plasmatic adenosine and adenosine-deaminase were performed., Results: Adenosine test was positive in 15 patients and negative in 14 patients. Patients with positive adenosine test had lower adenosine levels compared to patients with negative test (8.86 ± 2.07 ng/ml vs. 15.18 ± 2.14 ng/ml, p = .04). No difference was observed in adenosine deaminase levels (16.35 ± 2.20 IU/l vs. 13.20 ± 2.48 IU/l, p = .40). There was a negative correlation between adenosine level and AVB duration during adenosine test (p = .04; R 2  = 0.22). Patients with positive adenosine test had more frequent asystole during ILR monitoring than patients with negative test (9 pts vs. 1 pt, p = .005). Adenosine levels were lower in patients with asystolic syncope on ILR compared to vasodepressor syncope 8.20 ± 2.86 ng/ml versus 13.27 ± 7.26 ng/ml, p = .05)., Conclusions: Patients with positive adenosine test have decreased production of endogenous adenosine compared to patients with negative adenosine test. Positivity of adenosine test and low adenosine level in the peripheral blood were associated with more frequent asystolic episodes during ILR monitoring., (© 2022 Wiley Periodicals LLC.)