Objective: The aim of the study was to investigate endothelial function in treatment-naïve polymyalgia rheumatica (PMR) patients and its modification during steroid therapy, in relation to changes in clinical and laboratory parameters.Method: This prospective observational study involved patients with a new diagnosis of PMR according to provisional American College of Rheumatology/European League Against Rheumatism 2012 criteria, who were naïve to steroid therapy, and control subjects matched for age, gender, and comorbidities. All participants underwent clinical and vascular ultrasound evaluations at baseline and after 1, 3, 6, and 12 months of steroid therapy. For the study of endothelial function, we evaluated the brachial artery reactivity, which has emerged as the most well-established technique used in adults, by assessing flow-mediated dilatation (FMD), which measures the endothelium-dependent vasodilatation.Results: Sixteen newly diagnosed PMR patients were compared with a population of 16 matched controls. FMD values in all subjects showed an inverse correlation with the values of erythrocyte sedimentation rate and C-reactive protein. At baseline, the FMD of PMR patients was significantly lower than controls and remained significantly lower with respect to controls until the sixth month of therapy, despite a clinical improvement already being evident after 1 month of therapy.Conclusions: PMR is characterized by an important chronic subclinical inflammatory component. This pilot study demonstrates that affected patients show endothelial dysfunction that slowly responds to steroid therapy. Further studies are needed to investigate the clinical relevance of these observations and, in particular, to monitor the cardiovascular risk profile of PMR patients.