Abbreviations & AcronymsObjectives: To investigate the characteristics of Klinefelter syndrome in a consecutive series of men consulting for sexual as well as fertility problems in Korea. Methods: A total of 179 men with non-mosaic 47,XXY Klinefelter syndrome of the 1876 azoospermic males who visited the fertility center of our university hospital between January 2002 and January 2010 were included in this analysis. Their semen characteristics, hormone profiles, size of the prostate gland , and education level were assessed and compared with an age-matched control group of 218 fertile males. Additionally, a histological evaluation was carried out. Results: Prostate size, testicular volume, semen volume and symptom severity showed statistically significant differences in both groups (P < 0.0001). As the severity of signs and symptoms worsened , the education level decreased. Each degree of signs and symptoms was associated with a different level of education, as well as with significant differences in the level of testosterone and testicular volume (P < 0.05). Spearman's correlation coefficient showed the severity of signs and symptoms was associated with testosterone (P < 0.0001). Conclusions: Testosterone has the strongest negative association with the severity of signs and symptoms in patients with Klinefelter syndrome. It influences the reproductive capacity, as well as the manifested signs and symptoms of hypogonadism. Furthermore, it is also associated with various aspects of life in these patients.