We report two cases of low-grade glioma in which multiple cellular components, including cells with dense-core granules consistent with "neurocytes," were identified on electron microscopic studies. The first patient was an apparently normal boy until the onset of seizures at age 10 months. Initially, the seizures improved with phenobarbital treatment, but good seizure control was never achieved. Computed tomographic scan at age 23 months showed a calcified, nonenhancing left parietal mass. This tumor was composed of sheets of cells with clear cytoplasm and round to oval nuclei. Mucinous intercellular material stained positively with periodic acid-Schiff, mucicarmine, and alcian blue stains. Foci of calcification were evident. The second patient was a 13-year-old boy with a left parasagittal parieto-occipital mass who presented with a 4-month history of seizures and declining school performance. The tumor was composed of sheets of astrocytes with dark, hyperchromatic, pleomorphic nuclei in a fibrillary and microcystic background. The tumor contained the pleomorphism seen in the adult variant of pilocytic astrocytoma, as well as the microcystic component seen in the juvenile variety. Ultrastructurally in both cases, there were occasional tumor cells having round to oval nuclei with moderate amounts of cytoplasm containing 150- to 250-nm-diameter dense-core granules. These cells were admixed with the majority of tumor cells, which in case 1 had the ultrastructural features of astrocytes and oligodendrocytes and in case 2 had features of protoplasmic or pilocytic astrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)