OBJECTIVE: To show the importance of knowing diseases to make a correct diagnosis. Extranodal natural killer (NK)/T cell lymphoma, nasal-type is the most common cause of the syndrome known as "lethal midline granuloma." It is an extranodal lymphoma, usually with a NK cell phenotype and positivity for Epstein-Barr virus (EBV). The pathogenesis is poorly understood. CASE REPORT: A. M., a male 17 years of age, presented with an intranasal lesion on the right side that was rapidly growing and resulted in unilateral obstruction. He received treatment in October 2011, complaining mainly of pus, necrotic areas, and a fetid odor. There was destruction of the nasal dorsum and septal areas from necrosis. Videofibronasolaryngoscopy revealed an invasive lesion in the right nostril reaching the nasal septum, and a septal perforation with necrosis and crusts. The first biopsy and debridement were performed in October 2011. The pathology was inconclusive, and immunohistochemistry was not performed. A sinus CT scan revealed minimal bone destruction. A fungal culture, Leishmaniasis immunofluorescence, and sputum AFB were negative. The patient was referred to Santa Casa of BH in January 2012 for involvement of the hard palate and jaw at the midline. A daily fever, 15 kg weight loss, malaise, and an intense fetid odor were present. A sinus CT scan showed an absence of significant bone destruction. The patient was admitted for biopsy. PCR performed for Leishmaniasis, blastomycosis, Mycobacterium tuberculosis, and Mycobacterium leprae were all negative. A test for EBV was not performed. Immunohistochemistry was performed in February 2012 after the third biopsy, and was indicative of NK/T cell lymphoma, nasal-type. The patient was subsequently hospitalized with a progressive worsening in general condition, and specific treatment failed. Death occurred in March 2012. CONCLUSION: Extranodal NK/T cell lymphoma, nasal-type is the most common cause of the syndrome known as "lethal midline granuloma." Because of its aggressive behavior, it is important to make a correct and rapid diagnosis for correct treatment.