<p>Pituitary lesions have been sporadically reported in captive chimpanzee (<i>Pan troglodytes</i>) populations. These lesions can be functional (hormone secreting) or non-functional. Functional lesions produce different clinical syndromes depending on the hormone produced. Previous reports of pituitary adenomas in chimpanzees have not characterized the tumors through clinical syndromes or by immunohistochemical features of the lesions. A retrospective review of necropsy and medical records from the Chimpanzee Care Center (CCC) at The University of Texas MD Anderson Cancer Center were queried to identify chimpanzees with a diagnosis of a pituitary lesion. These cases were evaluated using reticulin staining to distinguish between hyperplasia and neoplasia and an immunohistochemical panel for adrenocorticotrophic hormone, prolactin, growth hormone, and thyroid stimulating hormone to assess hormone expression. In accordance with recent changes by the World Health Organization that reclassified pituitary adenomas as pituitary neuroendocrine tumors (PitNETs), the lesions were diagnosed using the updated terminology. Ninety records were reviewed and 18 cases (20%) had pituitary lesions: 4 pituitary cysts, 5 hyperplastic lesions, and 9 PitNETs. The median age of the cases was 44 [Range: 27.5–54 years of age]. Most cases were in females (16/18; 88.9%). Clinical signs of galactorrhea and elevated prolactin levels (&gt; 40 ng/mL) were reported in five animals diagnosed with plurihormonal hyperplasia (3/5), mammosomatotroph hyperplasia (1/5), and a lactrotroph PitNET (1/5), which indicates these lesions were functional. In the remaining cases, immunohistochemistry identified six ACTH + PitNETs, 1 plurihormonal PitNET, and 1 PitNET that was unclassifiable.</p>