PurposeTo detect the testosterone deficiency syndrome in Mongolian men over 40 years old with erectile dysfunction (ED).Materials and MethodsTotal of 309 males over 40 years of age who received medical care at the ADAM Urology and Andrology Clinic from 2010 to 2011 were included in this study. An approval from the Ethics Committee of the Ministry of Health of Mongolia was obtained, and each study participant signed a consent form at the beginning of the study. The participants were assigned to either an ED group or a control group, depending on the results of the international index of erectile function (IIEF)-5 questionnaire. The ED group was further divided into three groups (moderate, severe, and very severe) based on the level of ED. The total testosterone (TT) levels were determined in the blood serum using a competitive enzyme-linked immunesorbent assay (ELISA) analytical system UBI Magiwel™ Testosterone Quantitative test, and free testosterone (FT) calculated as described by the Vermeulen calculation. Test samples were collected between 8:00 and 11:00 am in the mornings and testosterone deficiency syndrome was diagnosed based on the International Society for the Study of the Aging Male guidelines, particularly, if TT was ≤3.46 ng/ml or free testosterone FT was ≤0.072 ng/ml.ResultsED of moderate, severe, and very severe levels was diagnosed in 199 (64.41%) out of 309 participants. There was an inverse relationship between the main IIEF-5 score and age (r=-0.380, p<0.01). The average TT was 5.75±2.316 ng/ml and FT was 0.091±0.0084 ng/ml. Compared to the ED group, the control group had a higher TT level: 5.6440±1.177 ng/ml and 5.812±2.316 ng/ml, respectively. In the control group, the FT level was 0.061±0.0084 ng/ml, whereas it was 0.041±0.0076 ng/ml in the ED group.ConclusionsOur study showed that most of the aging males who came to the clinic had moderate to very severe ED (64.55%). The levels of TT (5.644±1.177 ng/ml) and FT (0.041±0.0036 ng/ml) were significantly lower in ED patients (p<0.05). The testosterone deficiency syndrome was detected in 24.27% of the ED group.