Background. Either high or low values of adenosine blood level (ABL) can differentiate some forms of neurally-mediated syncope (NMS). A rapid method of measurement has recently been developed. The aim of the present study was: 1) to compare ABLs in an unselected population of consecutive patients referred for evaluation of suspected NMS syncope and in healthy controls; and 2) to assess the relative prevalence of low and high adenosine forms among an unselected syncope population Method. Whole blood was collected after finger puncture and its concentration was measured by liquid chromatography /mass spectrometry (LC-MS/MS). Results. Among 89 control subjects, the median ABL value was 0.54 µM (IQR 0.46-0.65). The lowest 5% and the upper 95% percentile were 0.40 and 0.80 µM respectively. Compared with healthy subjects, the 146 syncope patients showed on average a higher median ABL value (0.63 [IQR 0.45-0.73, p=0.04) and a larger distribution of values. Low ABL values below the 5th percentile were observed in 28 (19%) patients and in 5 controls, p=0.003 and high ABL values were observed in 26 (18%) patients and 5 controls, p=0.009. Conclusions. ABL is different in patients with suspected NMS than in healthy subjects. Patients with low and high adenosine values account for 19% and 18% of the general population. Thus, low and high ABL limits, as defined in this study, may help to define the purinergic profile of an unselected subjects with a clinical diagnosis of suspected NMS.