Summary We report 3 new patients with sinonasal renal cell–like adenocarcinoma (SNRCLA). One case submitted in consultation demonstrated robust carbonic anhydrase IX (CA-IX) expression, leading us to a broader inquiry of CA-IX and carbonic anhydrase II (CA-II) expression in other SNRCLA, Schneiderian tissues, and histologic mimickers. Robust cytoplasmic and membranous CA-IX expression is demonstrated in 6 of 7 SNRCLAs; CA-II expression was demonstrated in 2 of 5 cases. Robust, diffuse CA-II expression is demonstrated throughout sinonasal seromucinous glands in all 10 normal Schneiderian samples. CA-IX is also expressed in all normal sinonasal samples, albeit focally. The closest salivary mimic to SNRCLA is hyalinizing salivary clear cell carcinoma; only focal CA-IX expression was demonstrated in 1 of 2 cases studied. Carbonic anhydrase expression in Schneiderian tissue speaks to its role in regulating the ion concentration of sinonasal secretions and may also explain the origin of this rare tumor.