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Secondary non-Hodgkin lymphoma of the ethmoid sinus after temozolomide. The paranasal sinuses are rarely the site of malignancy, especially non-Hodgkin lymphoma. In such cases, the ethmoid sinus is the second most frequently involved paranasal sinus. Diagnosis of these malignancies is difficult because the early symptoms often mimic benign sinus pathology. Thus, most cases are diagnosed at an advanced stage, and their prognosis is poor. Here we describe the case of a 58-year-old man with a secondary high-grade B-cell non-Hodgkin lymphoma of the ethmoid sinus. This malignancy was diagnosed two years after the patient had received treatment with temozolomide for a glioblastoma multiforme. This case highlights that malignant tumours of the paranasal sinuses should always be included in the differential diagnosis of sinus disease. Additionally, patients treated with temozolomide should receive regular follow-up care including vigilant evaluation for secondary tumours, such as non-Hodgkin lymphoma.