Mucocele is a chronic, expansil and cyst-like lesion of the paranasal sinuses which contained sterile, mucoid secretions. Secondary infection may lead to the development of a pyocele. The mucoceles are most frequently found in the frontal sinus. The pathogenesis of primary mucoceles remains uncertain. Causatives factors of secondary mucoceles include chronic inflammation, trauma, allergy and obstruting tumors. Authors experienced a case of infected mucocele in bilateral frontal sinuses. A 52-year old male was admitted to Chung Ang University Hospital on Dec. 8, 1986 with Cliebconplaint of healacbe. X-ray findings showed total clouding of the entire paranasal sinuses and bone erosion with reactive marginal sclerosis of both frontal sinuses. Modified Lynch operation was performed and postoperative course was satisfactory.