BACKGROUND: Biopsy of the central nervous system (CNS) has been reported previously using relatively large-caliber (12-17-gauge) biopsy instruments with or without stereotactic guidance. In this series, fine-needle aspiration biopsies (FNABs) were performed using a smaller diameter needle to evaluate mass lesions of the CNS. METHODS: One hundred thirty FNABs were performed freehand under computed tomography (CT) guidance without stereotactic instrumentation guidance using a 22-gauge needle. RESULTS: A definitive diagnosis was rendered in 97 of 130 FNABs (75%), including glioblastoma multiforme (GBM) (n = 33 biopsies); anaplastic astrocytoma (n = 14 biopsies); metastatic carcinoma (n = 13 biopsies); low-grade astrocytoma (LGA) (n = 10 biopsies); lymphoma (n = 7 biopsies); oligodendroglioma (n = 5 biopsies); reactive gliosis (n = 2 biopsies); and abscess (n = 13 biopsies), with the infectious agent identified in 7 of 13 biopsies. Immunohistochemistry was employed in 51 biopsies to assist in determining either the cell type or the infectious agent. Indefinite diagnoses were rendered in 33 patients (25%), including LGA versus gliosis (n = 8 patients), necrosis and/or inflammation (n = 3 patients), nondiagnostic material (n = 17 patients), suspicious for lymphoma (n = 2 patients), suspicious for GBM (n = 2 patients), and high-grade neoplasm not otherwise classified (n = 1 patient). There was no morbidity or mortality attributed to the procedure. CONCLUSIONS: FNAB of CNS mass lesions established a diagnosis based on cytologic interpretation in 75% of biopsies without morbidity or mortality. Because morbidity and mortality rates in CT-guided biopsies utilizing stereotactic techniques with large-caliber needles have been reported as high as 14% and 4.7%, respectively, freehand FNAB under CT guidance may be a preferred initial method for evaluating mass lesions of the CNS. Copyright 2003 American Cancer Society.