When tumor involvement of the frontal sinus occurs, it is usually the result of the direct spread of the mass from the nasal cavity or anterior ethmoid sinuses. Primary frontal sinus tumors are extremely rare. We describe a case of primary adenocarcinoma of the frontal sinus in a 53-year-old man. The patient refused surgery, so he was treated with chemotherapy followed by radiotherapy. At the 1-year follow-up, only a slight reduction of the swelling was noted. Thereafter, he was lost to follow-up. This case represents a typical example of a paranasal cancer extending beyond the bony margins, with a silent onset that simulated benign disease. The best hope for an early diagnosis of such a tumor lies in the greater use of computed tomography to assess chronic rhinosinusitis.