Apoptosis and cell proliferation were studied in 180 human neuroepithelial tumors (30 medulloblastomas, 30 intracranial ependymomas, 30 oligodendrogliomas and 90 astrocytic tumors, including 30 astrocytomas, 30 anaplastic astrocytomas and 30 glioblastomas). Apoptotic nuclei were detected by morphology and in situ end-labeling (ISEL) of DNA breaks. The frequency of apoptotic nuclei varied from oncotype to oncotype and their distribution in each oncotype was uneven. An apoptotic index (AI) was calculated; this was high in malignant tumors and in tumors of embryonal origin and lower in tumors of the glial series. The AI/mitotic index (MI) ratio was lower in malignant tumors and higher in benign tumors, suggesting a relationship between apoptosis and cell proliferation. There was no significant correlation of either AI or AI/MI ratio with either labeling index (LI) of Ki-67 clone MIB-1 or with survival. A trends towards low AI/MI ratio in tumors with high LI and short survival was observed. Apoptosis expresses cell loss in tumors, but it did not appear to be a prognostic factor.