A case of epidermoid carcinoma arising in an epidermoid cyst in the cerebellopontine angle is presented. Clinical features and CT appearance are discussed in comparison with those of benign epidermoid cyst. A 43-year-old man was admitted to the Department of Neurosurgery, University of Tokyo Hospital on April 14, 1983, with complaints of right facial numbness and weakness of six months' history. On neurological examination, sensation of the right half of the face was decreased in all modalities. Marked atrophy of the ipsilateral temporal muscle was also noted. Right facial paresis of peripheral type was evident. Gag reflex was decreased on the same side. Except for a slightly increased left deep tendon reflexes, there were no pyramidal tract signs. A CT without contrast material failed to show any abnormalities. A postcontrast CT demonstrated an irregular enhancement in the right cerebellopontine angle. The finding of asymmetry of the ambient cistern indicated minimum mass effect on the metrizamide CT cisternography. Suboccipital exploration of the right cerebellopontine angle was carried out on April 28, 1983. Leaving a part of the capsule indenting the pons between the roots of the fifth and the seventh nerve, we removed a white pearly tumor. Histological diagnosis was typical epidermoid cyst. He left the hospital one month later with signs of the right seventh and the eighth nerve. His postoperative course, however, was beyond our expectation. Over a few months following his discharge, left hemiparesis as well as horizontal and vertical nystagmus gradually developed. He was readmitted on November 10, 1983. A postcontrast CT revealed enlargement of the enhanced lesion filling the right ambient cistern.(ABSTRACT TRUNCATED AT 250 WORDS)