Twenty-seven patients with Klinefelter's Syndrome, aged 19-38 years were divided according to the basal testosterone (T) levels into sub-eugonadal (less than or equal to 3 ng/ml) and eugonadal (greater than 3ng/ml) groups. The pretreatment T level was 3.21 +/- 1.59 ng/ml. The LH and FSH levels, 14.54 +/- 6.68 mIU/ml and 21.51 +/- 10.74 mIU/ml respectively, were above the upper eugonadal range. Short-term hCG treatment stimulated T production significantly and a further increase was observed following long-term hCG treatment. In patients with sub-eugonadal levels of basal T, a greater relative increment of the T level was observed following the hCG stimulation but not in the absolute T increase. Thus, the assumption that in Klinefelter's patients, the low basal T levels and the relative refractoriness to hCG stimulation are secondary to chronic exposure to elevated LH levels, could not be supported. Higher FSH levels were associated with elevated plasma T levels (p less than 0.025). No such association was established with the LH.