Mayer-Rokitansky-Kuster-Hauser syndrome (MRKHS) is characterized by vaginal agenesis with variable Mullerian duct abnormalities. We report here a case of uterine adenomyosis which developed from a hypoplastic uterus in a patient with MRKHS. A 55-year-old postmenopausal woman visited a university hospital for pelvic mass. She had underwent vaginoplasty via the McIndoe procedure for MRKHS at 15 years of age. Pelvic magnetic resonance imaging showed a 5.4 x 4.8 x 4.7 cm mass suspicious for a uterine myoma. She received total abdominal hysterectomy with bilateral salpingo-oophorectomy, and neither the cervix nor endometrium was found grossly in the surgical specimen. The final histologic diagnosis was uterine adenomyosis.