Patient: Male, 30-year-old Final Diagnosis: SMARCB 1 deficient sinonasal carcinoma Symptoms: Swelling • epistaxis Clinical Procedure: — Specialty: Pathology Objective: Mistake in diagnosis Background: SMARCB1-deficient sinonasal carcinoma is a rare neoplasm with inactivation of the SWI/SNF complex, with an aggressive clinical course as most of the lesions present as advanced in pT3/T4 stages with frequent recurrence, and many patients succumb to the disease. Reported initially in 2014, the lesion has male predominance, with an age range of 19 to 89 years and predilection for the ethmoid sinus and nasal cavity. Histopathological findings show a proliferation of small- to medium-sized monomorphic basaloid cells with indistinctive cytoplasmic borders and round variably prominent nuclei with scattered cells that show rhabdoid morphology. Cytoplasmic vacuoles are common. It has similar morphological findings to a wide array of neoplasms in the sinonasal area. Case Report: We report a case of SMARCB1-deficient sinonasal carcinoma in a 30-year-old man referred to our hospital with a preliminary diagnosis of sinonasal adenocarcinoma, intestinal type. Computed tomography showed a huge destructive soft tissue mass in the left maxillary sinus, extended to involve the left nasal cavity with extension to the skull base and perineural spread along the foramen rotundum. Histological examination revealed a malignant basaloid neoplasm embedded in a myxoid stroma that showed loss of SMARCB1 stain. The patient was treated with induction chemotherapy using etoposide and cisplatin for disease control. Conclusions: SMARCB1-deficient sinonasal carcinoma is a rare neoplasm with an aggressive clinical course and high-grade behavior despite having uniform cytological features. This poses complex diagnoses, especially in small biopsies. Incorporating morphological findings with ancillary tests is required to identify this high-grade malignancy.