BACKGROUND Transradial coronary catheterization has emerged over the last years as a favorable catheterization practice, based on evidence that it is associated with less vascular complications and shorter hospital stays. However, access site crossover appears to be more frequent when the initial route is the transradial one, one of the main reasons being arterial spasm. We hypothesized that radial flow-mediated dilation (FMD) measurements could be used as a preprocedural method to assess the likelihood of arterial spasm. METHODS The study population consisted of patients scheduled for transradial diagnostic catheterization in whom ad hoc percutaneous coronary intervention (PCI) was performed. FMD was measured 1-2 days before PCI. The primary endpoint of the study was operator-defined (operators were blinded as to the FMD results) radial artery spasm. RESULTS A total of 172 patients (110 male, age 65.3 ± 9) were included. Radial artery spasm was recorded in 13 patients (7.6%). FMD showed a very significant univariate association with the occurrence of spasm (P < 0.001) and was the most important predictor of spasm in the multivariate logistic regression analysis (beta -3.15; P < 0.001), followed by baseline radial artery diameter (P = 0.04), the number of catheters used (P = 0.049) and the administered volume of contrast medium (P = 0.017). CONCLUSIONS Preprocedural FMD is a significant predictor of arterial spasm before elective transradial PCI. It is a low cost, safe, and feasible noninvasive modality, whose results might be taken into account when deciding on the vascular access route for an elective procedure, the size of sheaths or catheters to be used or the intensity of antispasm medication.