Objective: To present a case describing an incidentally discovered pituitary mass that was found to be a collision tumor containing 2 distinct histologic cell types: (1) a growth hormone (GH)-secreting pituitary adenoma, and (2) a gangliocytoma. Methods: Sellar gangliocytomas are very rare benign neuroblastic tumors that originate from the posterior pituitary. The majority are associated with pituitary adenomas. The coexistence of these 2 morphologically distinct tumors is known as a collision tumor, a rare disease entity of which the pathogenesis is not well understood. We present a case of a woman with an incidentally discovered pituitary mass that was found to be a collision tumor. Results: A 44-year-old woman presented with an incidentally discovered pituitary mass and was found to have elevated insulin-like growth factor 1 (IGF-1) levels. The patient underwent endoscopic transsphenoidal excision of the pituitary mass. Histopathology of the tumor revealed a mixed pituitary adenoma that stained for GH and prolactin, and a gangliocytoma. Postoperatively, the patient developed temporary central adrenal insufficiency and permanent diabetes insipidus. Postoperative lab studies revealed a normal IGF-1 level, and a magnetic resonance imaging scan showed no residual or recurrent tumor. Conclusion: While sellar gangliocytoma is a rare lesion, when it is present, a co-existing pituitary adenoma should also be suspected. Further investigation should be done to determine the pathophysiology of these collision tumors, which could be beneficial in guiding diagnosis and treatment. (AACE Clinical Case Rep. 2019;5:e247-e249) Abbreviations: GH = growth hormone; IGF-1 = insulin-like growth factor 1; PRL = prolactin e248 Collision Tumor of the Sella, AACE Clinical Case Rep. 2019;5(No. 4)