Objective: To assess arterial stiffness in a cohort of hypogonadal males and to investigate the effect of testosterone replacement therapy on arterial properties in this specific group. Design: Eighteen male patients with untreated acquired hypogonadism due to either adult-onset idiopathic hypogonadotropic hypogonadism (n=9) or pituitary tumor (n=9) and 12 age-, sex, and weight-matched eugonadal healthy controls were recruited for the study. Arterial properties, plasma glucose, lipid profile, total, and bioavailable testosterone (BT) levels were measured in fasting state. In the hypogonadal subjects, the effect of transdermal testosterone replacement therapy on arterial properties was studied by repeat noninvasive measurements at baseline, as well as 48 h and 90 days following the initiation of treatment. Methods: Arterial stiffness was evaluated using applanation tonometry and pulse wave analysis by three different standard devices that assess various measures of arterial stiffness: pulse wave velocity (PWV), augmentation index (AIx), and large/small artery compliance (C1 and C2). Results: Age- and blood pressure-adjusted PWV was significantly higher in hypogonadal men (8.90± 2.29 vs 6.78±1.16 m/s in the control group; P=0.025). Testosterone therapy increased BT level from 2.01±1.04 to 4.68±2.43 and 7.83±6.2 nmol/l after 48 h and 3 months respectively (P=0.001). PWV decreased from 8.9±2.29 to 8.24±1.39 and 8.25±1.82 m/s after 48 h and 3 months of treatment respectively (P=0.03). Conclusions: Male hypogonadism is associated with increased PWV, which is rapidly but incompletely ameliorated by normalization of circulating testosterone levels.