A dermoid cyst of the posterior fossa in a 73-year-old man is reported. The presentation of the cyst was unusual because of the age of the patient, the spontaneously hyperdense aspect of the mass on CT, the partial rim enhancement of the lesion, and the presence of perilesional edema. On pathologic examination, the cyst contained small amount of fat, hairs, necrosis, and small areas of hemorrhage. The amount of hemorrhage found could not explain the spontaneous hyperdensity of the lesion found on CT. The hyperdensity may be related to high protein content of the lesion.