Background: To evaluate the involvement of EGFR signalling and HPV infection in a cohort of inverted sinonasal papilloma (ISP) and sinonasal squamous cell carcinoma (SNSCC) and their value for prognosis and clinical treatment. Methods:We analysed 55 ISP, 14 SNSCC associated with ISP (SNSCC-isp) and and 60 SNSCC not associated with ISP (SNSCC-novo) for EGFR gene mutation and copy number gain, protein expression of EGFR and phosporylated EGFR (pEGFR), and HPV-infection and KRAS mutation. Findings were correlated to clinico-pathological and follow-up data. Results:We found EGFR exon 20 mutations in 38% (7/18) ISP, in 50% (6/12) SNSCC-isp and in 5% (1/19) SNSCC-novo. EGFR was expressed in 92% of ISP, while pEGFR was observed in 54% (21/39). SNSCC-isp and SNSCC-novo demonstrated comparable expression of EGFR (57% and 33%) and of pEGFR (44% and 38%). We observed an inverse relation between EGFR exon 20 mutation and pEGFR expression. Four of 39 (10%) ISP carried HPV-16. Oncogenic HPV was detected in 3/12 (25%) SNSSC-isp and in 1/8 (13%) SNSCC-novo. KRAS mutations were not detected in any of the samples. HPV infection was inversely correlated with pEGFR expression but not with EGFR mutation. ISP with EGFR activation by mutation or by phosphorylation had longer ISP-free survival, however, neither EGFR exon 20 mutation, pEGFR expression nor HPV infection demonstrated prognostic value in SNSCC.Conclusions: EGFR exon 20 mutation is frequent in ISP and SNSCC-isp, while activation of EGFR through phosphorylation also plays an important role. Our data indicate that a large proportion of SNSCC patients could benefit from therapy with modern EGFR inhibitors.