Fluid aspirated from a subarachnoid cystic lesion that covered and compressed part of the left frontal lobe was examined cytologically and compared with histologic sections of the cyst wall. The fluid contained epithelial and histiocytelike cell populations. The epithelial cells were tall columnar, occurring singly or in clusters or sheets. Many cells were ciliated and their cytoplasm showed characteristic refractile granules. The differential diagnosis of this rare type of subarachnoid cyst and the mechanism of the development are discussed. Cytologic evaluation of the fluid of the subarachnoid cysts is potentially a more accurate method of classification of these lesions than is random biopsy of the cyst wall. It is of particular importance in cases with a history of growth, in which the progressive expansion results in attenuation of the diagnostic epithelial lining of the cyst.