Pituicytoma is a rare, poorly characterized tumor of the sella and suprasellar region that is distinct morphologically from other local tumors and is thought to be derived from neurohypophyseal pituicytes. Clinical data, neuroimaging studies, and microsections were reviewed from nine such low-grade gliomas. Immunostains for glial, neuronal, and proliferation markers were performed on all nine tumors and six control neurohypophyses. Three tumors were studied ultrastructurally. Six men and three women, age 30 to 83 years (mean, 48 years), presented with visual symptoms, headache, or hypopituitarism. Magnetic resonance images showed solid, discrete, contrast-enhancing masses, four within the sella and five in the suprasellar space. The tumors consisted of sheets and/or fascicles of plump spindle cells with slightly fibrillar cytoplasm and slightly pleomorphic, oval-to-elongate nuclei with pinpoint nucleoli. Extracellular mucin was prominent in one tumor. Rosenthal fibers, granular bodies, and Herring bodies (granular axonal dilatations characteristic of the normal neurohypophysis) were lacking. Mitoses were rare or absent. MIB-1 labeling indices were low (0.5-2%). Tumor cells were strongly reactive for vimentin and S-100 protein, variably positive for glial fibrillary acidic protein, and nonreactive for synaptophysin and neurofilament protein. Cytoplasm varied in electron density and contained intermediate filaments. Neither meningothelial nor ependymal features were noted. Two tumors recurred at 20 and 26 months after subtotal resection, but none of the six completely resected tumors have done so. Pituicytomas are discrete, largely noninfiltrative low-grade gliomas of the sellar region that occur in adults. Their histologic appearance is distinct from pilocytic and ordinary, infiltrative astrocytomas. The distinction between pituicytoma and normal neurohypophysis is aided by the latter's content of axons, Herring bodies, and perivascular anucleate zones rich in axonal terminations. Although curable by total excision, subtotal resection can be associated with recurrence.