Background Hemihypertrophy is a rare clinical presentation of Cushing's disease. It has been described in three newborns with massive enlargement of adrenal glands in presence of other congenital abnormalities and in a 17-year-old female with pituitary adenoma with a genetic diagnosis of Beckwith Weidman syndrome. We describe a case of Cushing's disease with lower extremity hemihypertrophy in the absence of features suggestive of an underlying syndrome. 1,2 Clinical Case: A 12-year-old male with no significant past medical history presented with poor growth and rapid weight gain. In addition, he had hemihypertrophy of left lower extremity interfering with his physical activity. Physical exam showed Tanner Stage 1 for pubic hair and testicular volume of 4 ml bilaterally, with no signs of hirsutism, bruising or purplish striae. Initial work up showed normal thyroid hormone and growth factor levels. Midnight salivary cortisol levels were 238 ng/dl and 51 ng/dl respectively (range <100 ng/dl). One mg dexamethasone suppression test showed suppressed cortisol level of 1 mcg/dl. At the subsequent visit seven months later, he had gained 13.8 kg and grown 0.9 cm in height. Physical exam showed facial plethora and buffalo hump, which had not been present at the initial visit. Left and right leg circumference above the knee were 22 inches and 20 inches respectively. Repeat midnight cortisol levels on two separate occasions were 251 ng/dl and 441 ng/dl respectively. Repeat 1 mg dexamethasone suppression test showed suppressed cortisol levels of 1.2 mcg/dl. Because of clinical picture consistent with Cushing syndrome and elevated salivary cortisol levels, 24-hour urine cortisol levels were ordered that were 145 mcg and 276 mcg/24h respectively (range 1-45 mcg/24hours). ACTH was elevated at 65 pg/ml. CT scan of abdomen and pelvis was normal. Left leg ultrasound was normal. MRI of brain showed a 4 mm hypoenhancing lesion in the pituitary gland to the left of midline. A peripheral DDAVP stimulation test showed elevated ACTH and cortisol levels suggestive of pituitary origin. MRI of legs showed asymmetric fat deposition causing hemihypertrophy. Patient underwent transsphenoidal resection of pituitary adenoma with postoperative remission (postop cortisol nadir <1 mcg/dl). At follow up visit after surgery, patient had lost 0.2 kg, buffalo hump had disappeared, and left leg circumference had slightly improved. Genetic testing ispending. Conclusion This case demonstrates that hemihypertrophy can be a presenting symptom of Cushing's disease. References: (1)Brioude F, Nicolas C, Marey I, et al. Hypercortisolism due to a pituitary adenoma associated with beckwith-wiedemann syndrome. Horm Res Paediatr. 2016;86(3): 206-211. (2)Carney JA, Ho J, Kitsuda K, Young WF, Stratakis CA. Massive neonatal adrenal enlargement due to cytomegaly, persistence of the transient cortex, and hyperplasia of the permanent cortex: Findings in cushing syndrome associated with hemihypertrophy. Am J Surg Pathol. 2012;36(10): 1452-1463. Presentation: No date and time listed