Circulating endothelial progenitor cells (EPCs) and endothelial microparticles (EMPs) increase, respectively, in the attempt to repair the damaged endothelium and in response to endothelial dysfunction. Erectile dysfunction (ED) of arterial origin recognizes endothelial dysfunction as one of its main determinants and shares risk factors and physiopathological evolution with the metabolic syndrome (MetS). Tadalafil, selective inhibitor of phosphodiesterase V, long half-life, is used to treat erectile dysfunction, and several studies have already documented the beneficial effects on endothelial dysfunction. The aim of this paper was to evaluate the concentrations of EPCs and EMPs in patients with arterial ED and MetS, before and after tadalafil administration, and in healthy men. Thirty patients (47-54 years) with ED and MetS (ATP III 1999 criteria) and 17 healthy men (44-57 years) were selected. EPCs (CD45neg/CD34pos/CD144pos) and EMPs (CD45neg/CD34neg/CD144pos) blood concentrations were evaluated by flow cytometry before and after administration of tadalafil (20 mg) on demand for 3 months. After treatment, the patients were divided into responders and poor responders, according to their IIEF-5 score. Blood EPCs and EMPs. Before treatment, the percentage of EPCs and EMPs was significantly higher in patients with ED and MetS compared to healthy men. Treatment with tadalafil increased significantly EPCs in both responders and poor responders. The latter had significantly higher EPCs compared to responders, both before and after tadalafil. Before tadalafil, EMPs were higher, but not significantly, in poor responders vs. responders. No significant change occurred after tadalafil administration in both responders and poor responders. A significant positive correlation was found between EPCs and age, Body Mass Index (BMI), acceleration time, IMT and EDV; whereas a negative correlation was found with IIEF-5 score, PSV and resistance index. EMPs correlated positively with BMI, acceleration time and IMT and negatively with the IIEF-5 score. Tadalafil increased the percentage of EPCs in both responders and poor responders, suggesting the persistence of an adequate bone marrow response. The unchanged EMP concentrations after tadalafil suggest a reduction of the dysfunctional mechanism.