We report a case of 42-year-old male patient with hypogonadotropic hypogonadism. He suffered from general fatigue and erectile dysfunction after the treatment with transdermal fentanyl for chronic pain by traffic injury. Endocrine examinations and hormone stimulating tests showed that he had hypogonadotropic hypogonadism. Brain magnetic resonance imaging (MRI) showed no abnormal findings, and he had no past history of accounting for acquired hypogonadotropic hypogonadism. Therefore, his hypogonadism was diagnosed to be caused by opioid treatment. Although opioid-induced endocrine dysfunctions are not widely recognized, this case suggests that we should consider the possibility of endocrine dysfunctions in patients with opioid treatment.