Klinefelter syndrome is characterized by progressive testicular failure causing androgen deficiency and azoospermia in most patients. The aim of this study was to evaluate semen quality in consecutive patients with an additional X chromosome as compared with healthy males. Forty-seven males with non-mosaic 47,XXY (n = 40) or SRY-positive 46,XX male (n = 7) karyotypes aged 26.1 (range: 15.0-51.7) years participated. Semen quality was compared with 2136 (control group I) men from the general population aged 18.9 (17.9-28.6) years and with 349 fertile men (control group II) aged 30.9 (22.0-43.8) years. Semen volume adjusted for duration of abstinence was significantly smaller in the patients [2.0 (0.2-5.7) mL] when compared with control group I [3.1 (0.3-12.5) mL, p <0.0001] and group II [3.6 (0.6-12.5) mL, p <0.0001]. There was no difference in semen volume between 47,XXY and 46,XX males. All patients had azoospermia except two 47,XXY males aged 29 years who had sperm concentrations of 0.5 and 1.6 million/mL, respectively. We found significantly smaller semen volume in the patients when compared with controls, and the presence of motile spermatozoa in two out of 47 patients. The small semen volume supports the notion of 47,XXY patients being androgen insufficient despite serum testosterone levels within the normal range.