AbstractThymic neuroendocrine tumors associated with multiple endocrine neoplasia are only defined as carcinoid and are not associated with large‐cell neuroendocrine carcinoma (LCNEC). We report the case of a multiple endocrine neoplasia type 1 patient with atypical carcinoid tumors with elevated mitotic counts (AC‐h), an intermediate condition between carcinoid and LCNEC. A 27‐year‐old man underwent surgery for an anterior mediastinal mass and was diagnosed with thymic LCNEC. Fifteen years later, a mass appeared at the same site, which was determined to be a postoperative recurrence based on the pathological results of a needle biopsy and the clinical course. The patient's disease remained stable for 10 months on anti‐programmed death‐ligand 1 antibody and platinum‐containing chemotherapy. The needle biopsy specimen was submitted for next‐generation sequencing, which revealed aMEN1gene mutation, and after further examination, a diagnosis of multiple endocrine neoplasia type 1 was made. A re‐examination of the surgical specimen from 15 years prior showed that it corresponded to AC‐h. Although thymic AC‐h is classified as thymic LCNEC according to the current definition, our data suggests that a search for multiple endocrine neoplasia is warranted in such patients.