Aim: To clarify the immuno-active LH (i-LH) and bioactive LH (b-LH) responses and qualitative changes in the cir-culating LH to testosterone undecanoate (TU) injection. Methods: Eight men with Klinefelter’s syndrome were re-cruited for the study. They received crossover injections of TU at doses of 500 and 1000 mg. Serum i-LH and b-LHlevels before and at various time intervals after TU injection were measured and the serum i-LH, b-LH, b-LH/i-LH(B/I) and testosterone/sex hormone-binding globulin (T/SHBG) ratio in LH-responders and LH non-responders werecompared. Results: A parallel suppression of serum i-LH and b-LH was consistent with their overall high correlationbetween each other ( r = 0.84, P < 0. 001). Mean serum i-FSH levels were decreased by TU injection at both doseswithout dose-response effects. LH-responders had lower baseline serum i-LH and b-LH, and higher E_2 levels and T/SHBG ratio. There was a quantitative change in serum LH as induced by TU without qualitative change within LH-re-sponder Aim: To clarify the immuno-active LH (i-LH) and bioactive LH (b-LH) responses and qualitative changes in the cir- culating LH to testosterone undecanoate (TU) injection. Methods: Eight men with Klinefelter’s syndrome were re- They were given crossover injections of TU at doses of 500 and 1000 mg. Serum i-LH and b-LH Levels before and at various time intervals after TU injection were measured and the serum i-LH, b-LH, b LH / i-LH (B / I) and testosterone / sex hormone-binding globulin (T / SHBG) ratio in LH-responders and LH non-responders werecompared. Results: A parallel suppression of serum i-LH and b-LH were consistently with their overall high correlation between each other (r = 0.84, P <0.001). Mean serum i-FSH levels were decreased by TU injection at both doseswithout dose-response effects. LH-responders had lower baseline serum i-LH and b-LH, and higher E_2 levels and T / SHBG ratio. There was a quantitative change in serum LH as induced by TU without qualitative change wi thin LH-re-sponder