Hormone levels were measured in patients with Klinefelter's syndrome after treatment with 100 mg mesterolone cipionate (twice monthly). There was no difference in plasma testosterone and FSH levels in treated and untreated patients. The basal and maximum LH levels were lower, but remained raised. The urinary excretion of testosterone as measured by liquid gaschromatography was higher in treated patients after treatment was discontinued.-From these results it is concluded that in spite of reported decreases of plasma testosterone during therapy with mesterolone cipionate this drug does not lead to severe impairment of the endogenous hormone production after discontinuing treatment.