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Background: Insulinoma‐associated protein 1 (INSM1) has been reported to be a useful marker for diagnosing pancreatic neuroendocrine tumours (PNETs). However, INSM1 expression in endoscopic ultrasound‐guided fine needle aspiration cytology has not been examined. We evaluated INSM1 expression in the cytology of cases diagnosed with PNETs. Methods: We immunocytochemically stained INSM1 and Ki‐67 in 14 PNET cases, and according to the 2017 World Health Organisation criteria, seven PNET Grade 1 cases, four Grade 2 cases and three Grade 3/neuroendocrine carcinoma cases were identified. As a control for INSM1 and Ki‐67 expression, we used cytological specimens from 15 cases of pancreatic ductal adenocarcinoma. Results: In PNET cases, INSM1‐expressing tumour cells were identified in all cytological specimens of PNET. The median INSM1 expression rate in Grade 1 cases was 49.8% (mean ± standard deviation: 55.1 ± 12.5%, min: 39.3%, max: 74.1%), and in Grade 2 and Grade 3/neuroendocrine carcinoma cases was 81.1% (mean ± standard deviation: 77.6 ± 18.6%, min: 50.3%, max: 100%). However, there was no correlation between INSM1 and Ki‐67 expression (r = −0.15). The median expression rate in PNET cases was 64.3%, which was significantly higher than that in pancreatic ductal adenocarcinoma cases (P < 0.0001). Conclusion: INSM1 immunocytochemistry of cytological specimens obtained from endoscopic ultrasound‐guided fine needle aspiration cytology can accurately diagnose PNETs; therefore, INSM1 could be an important diagnostic tool in assessing therapeutic strategies, including molecular‐targeted therapy. Recently, neuroendocrine markers such as SYP, CGA, and NCAM have been used with high specificity and sensitivity for PNETs. However, there are similarities in cell structure and immunocytochemistry between pancreatic solid tumors. We reported INSM1 positivity in all histological specimens of PNETs examined including high grade PNETs. This can be considered a useful biomarker. [ABSTRACT FROM AUTHOR]