Four examples of a novel glioneuronal neoplasm are presented. All tumors affected adults (including two males and two females aged 25-40 years) as supratentorial, cerebral hemispheric masses with associated seizure activity and, in one case, symptoms of raised intracranial pressure and progressive hemiparesis. CT scans in two cases revealed hypodense masses without calcification. MRI scans at presentation demonstrated, in all cases, solid T1-hypointense and T2-hyperintense tumors with mass effect in one instance but no edema or contrast enhancement. Only one was relatively circumscribed on neuroradiologic study. All were infiltrative in their histologic growth pattern and predominantly glial in appearance, being composed mainly of fibrillary, gemistocytic, or protoplasmic astroglial elements of WHO grade II to III. Their distinguishing feature was their content of sharply delimited, neuropil-like islands of intense synaptophysin reactivity inhabited and rimmed in rosetted fashion by cells demonstrating strong nuclear immunolabeling for the neuronal antigens NeuN and Hu. These cells included small, oligodendrocyte-like ("neurocytic") elements as well as larger, more pleomorphic forms. Two cases contained, in addition, well-differentiated neurons of medium to ganglion-cell size. Proliferative activity was observed principally within the glial compartment; two cases contained mitotic figures and exhibited relatively elevated MIB-1 indices (6.8% and 8.2%). One of the latter progressed and proved fatal at 30 months following subtotal resection and radiotherapy. The three other patients are alive at intervals of 14 to 83 months, two tumor-free and one with extensive disease associated with the appearance of enhancement on MRI. This glioneuronal tumor variant may pursue an unfavorable clinical course.