Congenital cholesteatoma may originate in various locations within the temporal bone, including the petrous apex, the area of the geniculate ganglion, the jugular foramen, the middle ear, the mastoid process, and the squamous portion of the temporal bone. According to Nager, 1 the mastoid process is the least frequent. We report an unusual case of congenital cholesteatoma that originated in and was restricted to the mastoid process and presented as an expansile mass that compressed the cerebellum. CASE REPORTA 54-year-old woman presented at the University of Miami Ear Institute in 1993 with right neck pain and persistent instability. She had no previous ear surgery. The findings on physical examination were unremarkable, and a hearing test showed a symmetric sloping high-tone sensorineural hearing loss.High-resolution computed tomography (CT) of the temporal bones revealed an expansile lytic lesion in the right mastoid (Fig 1 ). Her mastoid pneumatization was well developed bilaterally. The middle ear, inner ear, attic, aditus ad antrum, and antrum were within normal limits. Magnetic resonance imaging (MRI) of the brain revealed a nonenhancing expansile lesion of the right mastoid, involving the mastoid tip and the posterior aspect of the mastoid air cells (Fig 2). The lesion had compressed the adjacent right cerebellar hemisphere and caused narrowing of the right transverse sinus. On T2-weighted images it was hyperintense, and on Tl-weighted images, moderately hypointense. The signal characteristics were those of a semisolid mass, not of a simple cyst.The patient was lost to follow-up until 1 year later, when she presented with similar symptoms. A subsequent CT without contrast showed no significant differences from the original findings.At surgery the mastoid was found to be filled by the cholesteatoma, which had almost completely destroyed the tegmen mastoideum, the posterior fossa bony plate, and the bony plate covering the sigmoid sinus. The sinodural angle had been invaded and the superior petrosal sinus had been exposed. However, Fig 1. Erosion and expansion of right mastoid. Axial computed tomogram ( 1 -mm-thick section, with gray scale adjusted for bone imaging) at level of epitympanum and internal auditory canal demonstrates erosion and expansion of mastoid, especially at medial margin (arrows), as well as destruction of normal bony septations between mastoid air cells. Internal auditory canal, bony labyrinth, and tympanic cavity are normal.