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To the Editor.— Alker et al's report "Radiology of a Large Cisterna Magna Cyst" (Archives36:376-379, 1979) describes an 18-year-old woman admitted for seizures and headaches. A pneumoencephalogram showed a huge cystic structure in the posterior fossa that was thought to be a dilated cisterna magna. They believed that the cyst was acting as a mass lesion exerting pressure on posterior fossa structures without causing hydrocephalus. At the time of operation, some evidence of "long-standing pressure" was noted. After surgery, the seizures and headaches disappeared. They mentioned that such marked enlargement of the cisterna magna is rare. We reported 1 the incidence of "mega cisterna magna" in 3,000 computerized tomography (CT) scans and found 11 such cases. One of our cases also had a thin skull in the area of the cyst. In two cases, surgical intervention gave no symptomatic relief. We believed it was most likely that the mega