Endothelial progenitor cells (EPCs), involved in the repairing mechanisms of vascular damage, are positively correlated to insulin-like growth factor I (IGF-I) concentrations in healthy adults. However, the levels of EPCs and their role in acromegalic patients have never been investigated.We conducted a cross-sectional study in order to assess the levels of the different phenotypes of circulating EPC in acromegalic patients.The study was performed at the Endocrinology Unit of Federico II University and at the Unit of Metabolic Diseases and Endocrinology of the Second University of Naples. Fifty-five acromegalic patients and 65 healthy controls were studied. EPCs were assessed by flow cytometry and IGF-I by immunoradiometric assay.Compared with subjects of the control group, acromegalic patients showed significantly higher levels of EPCs phenotypes expressing KDR antigen [KDR+, cells per 106 events, median and interquartile range, 44 (28-67) vs 23 (13-40), p=0.006; CD34+KDR+ 25 (18-38) vs 12 (8-17), p<0.001; CD133+KDR+ 17 (13-30) vs 8 (6-12), p<0.001; CD34+KDR+CD133+ 16 (12-25) vs 8 (6-10), p<0.001]. There was a positive correlations between CD34+KDR+CD133+ cells count and IGF-I in acromegaly group (r=0.79, p<0.001).Acromegalic patients show higher circulating EPCs levels expressing KDR, positively correlated with IGF-I, suggesting a role for IGF-I in regulating the expression of this surface marker in the early phase of EPCs differentiation.