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Background and Aims: High-grade gastrointestinal neuroendocrine neoplasms (GI-NENs) are divided into well-differentiated G3 neuroendocrine tumors (NETs G3) and neuroendocrine carcinomas (NECs), having identical cut-offs of proliferation, but different biomolecular origins. This translates in distinct treatment choices. Our aim was to establish if p53/Rb1 immunohistochemical status in GI-NENs with Ki67 index >20% can predict the histopathological diagnosis., Methods: p53/Rb1 immunolabelling was performed on 42 cases of high-grade GI-NENs, diagnosed as NET G3, NEC and mixed neuroendocrine-non-neuroendocrine neoplasms (MiNEN) with NEC component. Immunolabeled slides were digitally scanned, with automatic quantification of p53 and Rb1, blind to the diagnosis., Results: The p53 positive percentage was stratified; two cut-offs were selected, naming the intervals as N (null, <1%), T (tumor, 1%-20%) and C (carcinoma, >20%). The Rb1 expression loss in >90% of neoplastic cells was considered mutational. NETs G3 mainly showed the T status (14/16, 87.5%), followed by N (1/16, 6.25%) and C (1/16, 6.25%); NECs and NEC components in MiNENs predominantly expressed the C status (19/26, 73.08%), followed by N (5/26, 19.23%) and T (2/26, 7.69%) (p<0.001, χ 2 =27.017). NET G3s showed positive expression for Rb1; 73.08% of NECs expressed negative Rb1 (p<0.001, χ 2 =21.351). NECs and NEC components in MiNENs showed Rb1 mutational status in 13 C cases (13/19, 68.42%), 4 N cases (4/5, 80%) and in both the T cases (p=0.002, χ 2 =11.187)., Conclusions: Our results highlight the correlations between the p53/Rb1 immunostainings and the histopathological diagnosis of high-grade GI-NENs. NECs and NEC components in MiNENs showed a p53 mutational status (0% or 21-100%) and predominantly negative Rb1 expression. NETs G3 showed a p53 wild-type status (1-20%) and retained Rb1 expression. These findings suggest that the differential diagnosis of high-grade GI-NENs may benefit from p53/Rb1 immunohistochemical tests in everyday practice.