Dipeptidyl peptidase-4 (DPP-4) inhibitors, such as saxagliptin, have gained a rapid growth in use in the treatment of type 2 diabetes mellitus in the past decade. Although they are considered to have a good safety profile, controversy exists regarding their potential to stimulate neoplasm growth. We report here a patient with metastatic carcinoid tumor. His disease was stable for several years with plasma serotonin level (which was used to monitor disease progression) in 700-800 ng/mL range. After initiation of treatment with saxagliptin, however, his serotonin level almost doubled (1358 ng/mL), concerning progression of the disease. After discontinuation of saxagliptin, serotonin level returned to baseline quickly, while other laboratory markers, such as complete blood count (CBC), comprehensive metabolic profile (CMP) with liver function tests (LFTs), and lactate dehydrogenase (LD), remained unchanged before, during, and after the treatment with saxagliptin. This temporal correlation suggests a possible interaction between the activity of carcinoid tumors and the use of DPP-4 inhibitors. Although we were not able to find any literature providing a direct evidence that saxagliptin alters progression of the carcinoid tumors, we recommend alternative management for the treatment of diabetes in patients with carcinoid or other neuroendocrine tumors.