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A case of large epidermoid located in the fourth ventricle is presented, and the patient's uncommon symptomatology of bronchial asthma-like episode is discussed. The value of magnetic resonance imaging (MRI) is also emphasized in the diagnosis of intracranial epidermoid. A 41-year-old male noticed nausea and vomiting on getting up in the morning about 5 years ago. This was followed by bronchial asthma-like dyspnea one year later. About one week prior to admission, headache and gait disturbance started. On neurological examination, he had choked disk and horizontal nystagmus at lateral gaze bilaterally. His gait was slightly ataxic. Computerized tomography (CT) showed a low density mass with a sharp and irregular margin in the mid-portion of the posterior fossa. That lesion was not enhanced with contrast medium. The MRI appearance was that of an inhomogenous and low signal intensity mass with a slightly irregular margin on T1-weighted spin echo (SE) sequences using TR500 msec/TR30 msec (TR500/TE30). The tumor extended into the aqueduct upward and the C1 level of spinal column downward. T2-weighted SE sequences using TR2000/TE90 showed an inhomogenous and high intensity mass with an irregular margin more apparent than in normal brain tissue. The patient was tentatively diagnosed as having a large fourth ventricle tumor. Suboccipital craniectomy was carried out on 4, March, 1988. The tumor was removed totally and histologically, it turned out to be epidermoid. He was discharged without neurological deficit 2 months after surgery. First, with respect to clinical symptomatology, as specified by Bailey, it is characterized by difficulty in standing or walking, vertigo, and less constantly, psychic disturbance.(ABSTRACT TRUNCATED AT 250 WORDS)