Non-Hodgkin's lymphoma of the sinonasal tract is an uncommon lesion, representing 1.5 to 15% of all lymphomas. Most cases of primary non-Hodgkin's lymphoma of the sinonasal tract occur in the maxillary sinus, ethmoid sinus, and nasal cavity; its occurrence in the frontal sinus is extremely rare. We report a case of primary type B non-Hodgkin's lymphoma of the frontal sinus in a 43-year-old man. The patient complained of frontal headaches that had not improved with analgesic drugs, and he presented with a frontal bulge that involved the left upper eyelid; the bulge had progressively enlarged over a 3-month period. A biopsy of the mass identified the type B non-Hodgkin's lymphoma. Immunohistochemical study not only confirmed the histologic type of the tumor, it also provided some important information about the primary tumor site. Advances in immunohistochemistry have shown that type B non-Hodgkin's lymphoma is more common in North American and European patients, whereas subtype T is more common in Asians and in some Latin Americans. The treatment of this condition is still controversial, but the combination of radiotherapy and chemotherapy has yielded the best results in all stages of the disease.