Background and Objective: Early detection of testosterone deficiency in middle aged and aging male improves their quality of life and prevents testosterone relating disorders.There is not enough evidence based study related testosterone deficiency syndrome and erectile dysfunction in Mongolia.Purpose of our study is investigating TDS and erectile dysfunction among men over 40 years old.We conducted questionnaires of AMS(Aging Male Symptom)scale and erectile function levels and forms of sexual function by IIEF-5 score.And we studied the relation of erectile dysfunction.To determine to testosterone deficiency by serum level of total testosterone less sensitive than free testosterone.To detect the testosterone deficiency syndrome in aging males with erectile dysfunction.Materials and Methods: A total of 309 males over 40 years of age enrolled this study.An approval of the Ethical Committee of MOH was obtained.Each participant was assigned to either an erectile dysfunction(ED)group or a control group depending on results of the IIEF-5 questionnaire.The ED group was further divided into three groups(moderate,severe and very severe)based on a level of ED.The total testosterone(TT)levels were determined in blood serum,using a competitive ELISA analytical system and free testosterone(FT)calculated as described by Vermeulen.Test samples were collected between 8:00-11:00 am.The biochemical diagnosis of TDS was based on ISSAM guidelines,particularly,if TT was ≤3.46 ng/mL or free testosterone FT was ≤0.072 ng/mL.Results: ED of moderate,severe and very severe levels were diagnosed in 199(64.41%)out of 309 participants.There was an inverse association between an erectile function 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.Late onset hypogonadism detection by using calculated free testosterone was three times higher when using total testosterone.Testosterone replacement therapy improved erectile function,aging and reducing waist circumference and maintain testosterone in biological level.Conclusions: Our study showed that most of aging males had a moderate to very severe ED(64.55%).The level of TT(5.644±1.177 ng/mL)and FT(0.041±0.0036 ng/mL)was significantly lower in ED patients(P<0.05).The testosterone deficiency syndrome was detected in 24.27%of the ED group.