Investigation of Klinefelter syndrome among infertility men Background: Klinefelter syndrome (KS) is the most common sex chromosomal anomaly, which is estimated to be 1 in 500-1000 male live births and 2.1-28.4 % among infertility men. Klinefelter syndrome is defined as LD50.30 by International Classification of Diseases 11th Revision. So far, KS syndrome has not investigated among Mongolian infertility men. Materials and methods: We recruited participants who were diagnosed in Center of Infertility, Child and Maternal National Center from October 2017 to October 2019. These participants showed azoospermia and the clinical characteristics of KS. Clinical characteristics of KS were as the anthropometric values, testicle size and also gingival cells were checked by Sanderson method to detect Barr body. Cytogenetically, blood lymphocytes were incubated and evaluated by standard method of G-band using “Metasystem” for karyotyping diagnosis. Results: During the research period, we recruited 1636 male participants with infertility and out of them 145 people were shown the azoospermia and nine people had clinical characteristics of KS. We confirmed the diagnosis of KS in five cases out of these nine patients (3,4%). The mean age of these KS cases was 33.5±4.5. Gynecomastia was presented in 100% (n=5), no underarm hair in 80% (n=4) cases, sparse body hair in 20% (n=1) cases, no facial hair in 100% (n=5) cases, female-like pubertal hair was detected in 80% (n=4) cases. Mean size of testicles was as 4±1.25 ml in left and as 4±0.75 ml in the right. We found that karyotyping pattern of the cases was in 80% 47,ХХУ (n=3) and mosaicism in 20% 48,ХХХУ/47,ХХУ (n=1) and 20% 46,ХХ (n=1), and Barr body was detected in all 100% (n=5) cases. Conclusions: Azoosperm was detected in 8.9% among infertility men in our study. Klinefelter syndrome cases were detected in 3.4% among infertility Mongolian men.