Eosinophilic granulocytes in the CSF were observed in 94 of approximately 10,000 qualitative cytologic preparations. Those cases of eosinophilia which occurred in the context of a parasitic disease or a puncture-related hemorrhage were excluded. CSF eosinophilia exceeding 1% was found in 57.5% of the cases and 5% in 23.5%. Increased cell counts were observed in 67.7% of the cases; elevated CSF protein values, in 68% to 73%; blood eosinophils, in 10.4%. There was no reason to suspect a relationship between these findings and the number of eosinophils in the CSF.--Fifty-two percent of the cases involved inflammatory diseases of the nervous system; the 18 cases of abacterial inflammation of unknown etiology were particularly striking. In the remaining cases, eosinophils were found in conjunction with cerebral ischemia and hemorrhage, with tumors, and in a relatively high percentage of children (21%). The frequency of occurrence with drained or undrained hydrocephalus was striking. A review of the pathophysiological function of eosinophils indicated that revived or corpuscular antigens were present in all cases of CSF eosinophilia in which an eosinophilic reaction was induced. Nothing can be said at this time, however, concerning the classification of the antigens.