Although the origin of cardiac syndrome X (CSX) is still debated, endothelial dysfunction leading to reduced coronary microvascular dilatory response and increased coronary resistance is thought to have an important role in the pathogenesis. Erectile dysfunction (ED) is associated with risk factors resulting in endotelial dysfunction. Although the relationship between cardiovascular disease and ED has been well established; the relation between CSX and ED has not been extensively studied so far. We herein aimed to study ED in patients with CSX. The study was designed as a prospective case-control study. Blood samples were analyzed with respect to concentrations of low-density lipoprotein cholesterol, high-density lipoprotein cholesterol and triglycerides. The subjects answered the native language five-item version of the International Index of Erectile Function Questionnaire (IIEF)-5. Each question was scored from 0 to 5 with a maximum score of 25 denoting healty subjects. We investigated the IIEF-5 score in 51 men with CSX (mean age 48.2 +/- 6.4 years), 53 men with demonstrated coronary artery disease (CAD) (mean age 48.3 +/- 4.8 years) and 52 male controls with normal coronary arteries (mean age 47.2 +/- 6.0 years). Mean IIEF-5 scores were 19.88 +/- 3.07 for CSX group, 18.83 +/- 3.31 for CAD group and 21.40 +/- 2.94 for control group. IIEF-5 scores in CSX group were found to be significantly lower than the those of control group (P<0.001). There were no significant differences in IIEF-5 scores between CSX and CAD groups (P = 0.09). We have shown for the first time that patients with CSX have lower IIEF-5 scores compared with controls with normal coronary angiograms. This study suggests that ED and CSX may be different manifestations of a common underlying vascular pathology and vasculogenic ED is frequently seen in CSX at least as much as in CAD.