Malignant transformation of a neurilemmoma is an exceedingly rare event. We describe a case of intrathoracic ancient neurilemmoma undergoing a malignant change in a 39-yr-old man. The patient presented with right flank and chest pain for several months. Plain radiography and CT scan of the chest showed a soft tissue mass lesion at the extrapleural space with erosion of surrounding ribs at the right basal lung area. The excised mass was encapsulated and measured 4.5 x 3.5 x 2.3 cm. The cut surface showed grayish-white and glistening with a focal cystic change and hemorrhage. Necrosis was not seen. Histologically, the tumor showed the features of classic neurilemmoma composed of the Antoni type A and B areas with perivascular hyalinization. In addition, obviously histo-logically malignant foci manifested by presence of markedly increased cellularity with fascicular arrangement, active mitotic activity, hyperchromasia, and gradual loss of original neurilemmomatous feature were noted.