We report a case of sinolith in the left ethmoid sinus of a 61-year-old man. The patient complained of nasal obstruction. Computed tomography revealed a small, calcified mass associated with a nasal polyp in the left ethmoid sinus. The antrolith and polyp were removed via endoscopic sinus surgery. Histopathological analysis of the antrolith revealed it to be bone-like in formation. The antrolith was about 1 cm in diameter. Infrared spectroscopy revealed that the antrolith contained protein (45 per cent), calcium phosphate (43 per cent) and calcium carbonate (12 per cent).