A case of a 23-year-old, nulliparous female with a very rare isolated adenomyotic cyst inducing severe dysmenorrhea was seen. Transvaginal ultrasonographic tomography and magnetic resonance imaging (MRI) showed a 3 x 3-cm cystic mass within the left anterior wall of the uterine corpus. The cystic space was filled with hyperintense fluid on T1-weighted images, which was surrounded by hypointense tissue beside the right uterine corpus on T2-weighted images. The case was preliminarily diagnosed using MRI as having cavitated rudimentary uterine horn. However, hysterosalpingography excluded the possibility of uterine anomaly. A hemorrhagic adenomyotic cyst measuring 3 cm within the left anterior wall of the uterine corpus was surgically removed. There was no evidence of diffuse adenomyosis uteri. Dysmenorrhea completely disappeared postoperatively.