Objective: Cytologic examination is a key element in the intraoperative assessment of central nervous system (CNS) mass lesions. However, differentiation of reactive, non-neoplastic tissue from low-grade neoplasms may be difficult. We reviewed cases on file to determine what cytologic features may be most helpful in distinguishing reactive tissue from cytologically similar appearing CNS tumors. Methods: Retrospective review of CNS biopsies on file between 1990 and 2002 with intraoperative cytologic evaluation and final diagnosis of reactive CNS tissue. H&E stained crush preparations were assessed on a scale of 0 (normal) to 3 in regards to tissue clumping, cellularity, presence of astrocytes with prominent cytoplasmic processes, granular eosinophilic background material, cellular heterogeneity, cytologic atypia, gemistocytes, Rosenthal fibers, foamy macrophages, lymphocytes, and neutrophils. The findings were compared with cytologic features found in representative cases of low-grade gliomas. Results: Materials were available for review in a variety of reactive biopsies ( n=33), low-grade diffuse astrocytomas ( n=11), pilocytic astrocytomas ( n=8), central neurocytomas ( n=3), and low-grade oligodendrogliomas ( n=3). Smears of reactive tissue showed a variety of features depending on the subtype of response present, including coagulative necrosis, subacute necrosis, radiation change, and nonspecific gliosis. The nonspecific gliosis most often showed low cellularity, evenly spread smears, retention of the eosinophilic background material, and variable accentuation of the astrocytic cytoplasmic processes. Oligodendrogliomas showed similar findings. Diffuse and pilocytic astrocytomas were usually distinguished from reactive tissue by the presence of coarse astrocytic cytoplasmic processes and loss of granular background material. However, some reactive biopsies showed similar features. Central neurocytomas were readily differentiated from other conditions due to its high cellularity, lack of cytoplasmic processes, and uniform cell population. Assessment of inflammatory cells, macrophages, cellular atypia, and Rosenthal fibers were not helpful in most cases. Conclusion: Cytologic features including the cellularity, resistance of the tissue to smearing, prominence of cytoplasmic processes, cellular heterogeneity, and retention of background granular eosinophilic material are helpful in most cases to differentiate reactive brain tissue from low-grade glial neoplasms. More cellular forms of gliosis may resemble diffuse or pilocytic astrocytomas and oligodendrogliomas.