Increased expression of the cell adhesion molecule, CD44 standard form (CD44s), has been associated with papillary epithelial tumors, and decreased expression has been linked to tumor invasion and metastasis. Sinonasal inverted papillomas (SIPs) are the most common papillary tumors of the sinonasal tract. This study tests whether the development of squamous cell carcinoma (SCC) in situ and invasive SCC in SIP is associated with altered expression of CD44s. Seventy-six specimens of SIP from 68 patients, 2 specimens of SIP with focal SCC in situ, and 10 specimens of invasive SCC arising in SIP were studied. Automated immunohistochemistry was performed for CD44s expression on paraffin-embedded tissue sections using mouse antihuman CD44 antibody. All 76 SIPs (100%) expressed CD44 (strong membranous staining, 83%; moderate staining, 12%; weak staining, 5%). Two (100%) of 2 SIPs with SCC in situ maintained strong expression in benign and severely dysplastic foci. Six (60%) of 10 SIPs with SCC showed complete loss of CD44s expression, while 4 (40%) of 10 cases of SIP with SCC showed weak expression. Two SIPs with SCC (20%) featured weak diffuse staining of the SCC component, and 2 SIPs with SCC (20%) featured weak focal staining of the SCC component. The non-SCC SIP components of the 10 SIPs with SCC uniformly featured intact membranous CD44 staining. As in other papillary epithelial neoplasms, the typical benign SIP features diffuse membranous CD44s expression. In cases of SIP developing an invasive SCC, CD44s expression in the SCC component is frequently lost.