We report an adolescent patient with Prader-Willi syndrome accompanying suppressible hypergonadotropism. The subject is an 18-year-old female. She was obese (body mass index: 35.7) and hypomyotonic with mental retardation. On endocrinological examination, a high serum LH concentration and hyperresponsiveness of luteinizing hormone (LH) to intravenously administered LH-Releasing Hormone (LH-RH) were observed, while the basal follicle stimulating hormone level was within the normal range. In addition, serum dehydroxyepiandrosterone sulfate (DHEA-S) was also increased. Following 2 mg dexamethasone administration for 7 days, serum LH and DHEA-S were almost normalized and hyperresponse of LH to LH-RH completely disappeared. The present study provides evidence that altered responsiveness to adrenal steroid may be involved in the establishment of hypergonadotropinism in an adolescent patient with Prader-Willi syndrome.