Congenital combined pituitary hormone deficiency (CPHD) is associated with deficiencies of anterior pituitary hormones. PROP1 gene mutations are often responsible for CPHD, but few such cases have been reported in Japan. This study describes a 37-year-old Japanese man with CPHD, treated with hydrocortisone, testosterone, and L-thyroxine, who was evaluated for adult growth hormone deficiency (GHD). Gene analysis revealed a previously unknown PROP1 mutation (R112X). After 10 months of recombinant human growth hormone (rhGH) administration, cortisol and urinary free cortisol levels were significantly lower than before therapy. This case underscores the importance of reassessing hypothalamic-pituitary-adrenal axis function in GHD patients, especially those with a PROP1 mutation, during rhGH therapy.