Abstract Disclosure: J. Muuse: None. R.J. Shah: None. P.A. Belany: None. Introduction: Complete Androgen Insensitivity Syndrome (CAIS) is a rare disorder of hormone resistance characterized by a female phenotype in an individual with an XY karyotype. Individuals with CAIS are born unambiguously female without any apparent abnormalities. They develop a short blind ending vaginal canal, with absence of Wolffian duct derived structures (vas deferens, seminal vesicles). Breast development advances regularly at puberty, while there is absent pubic and axillary hair, both due to peripheral conversion of testosterone to estrogen. Testes remain undescended in the abdominal cavity. Case: A 28 year old female with a past medical history of bipolar disorder presented to her primary care physician’s office with a chief complaint of fatigue. Blood work revealed an elevated testosterone level of 686, at which time she was referred to endocrinology for further work up. On further history gathering, she endorsed a life long history of amenorrhea. She saw an OB-GYN at 19 years old and was told she had a short vaginal canal. A pelvic ultrasound revealed no uterus and one left ovary. She was not given a diagnosis other than congenital absence of a uterus. She was lost to follow up until 28 years of age when she sought medical help for her fatigue. Physical exam was pertinent for a tall female with no signs of hirsuitism. Additional blood work revealed normal LH, FSH, and DHEA-S. A chromosomal analysis revealed XY. She was referred to gynecology for further management and assessment for orchiectomy. Discussion: This was an unusual case of CAIS, as this was diagnosed far past menarche. Most cases are diagnosed during puberty- which aids in an early approach towards the psychological and social well being of the individual as well as close follow up for evaluation of orchiectomy. Orchiectomy is recommended due to the increased risk of gonadal malignancy following puberty. The risk of developing malignancy increases with age, and has been reported to be between 15-22%. Thus it is important to identify this syndrome early in life to ensure this is addressed. It is followed by long term hormonal therapy with estrogen to maintain normal breast and bone development. This case highlights the importance of maintaining a high clinical suspicion of CAIS in females presenting with short vaginal canal and absence of a uterus in order to prevent malignancy and provide optimal psychological support to the individual. Presentation: Friday, June 16, 2023