bcl-2 protein (BCL-2) expression was immunohistochemically studied in 140 varied central nervous system tumors. The protein was most frequently expressed in neurinomas and ependymomas, and in normal ependymal cells and Schwann cells. Most pituitary adenomas could be classified into one of two subgroups, diffusely positive or diffusely negative tumors, while BCL-2 localized heterogeneously in normal pituitary glands. Although the protein was not detected in normal astrocytes, it was positive in reactive hypertrophic astrocytes observed in various pathological conditions. Similarly, astrocytic tumor cells often expressed BCL-2. Since low-grade astrocytomas more often exhibited the protein than malignant gliomas, the degree of BCL-2 expression appeared to be related to the degree of malignancy of the gliomas. On the other hand, 7 out of 17 recurrent gliomas and medulloblastomas showed an increase in the frequency of protein expression compared with specimens from initial treatments. One recurrent astrocytic tumor which demonstrated anaplastic change showed a decrease in the frequency of BCL-2-positive cells. It is concluded that the frequency of BCL-2 expression in CNS tumors is increased when the non-neoplastic counterparts of the tumors exhibit the protein. Although it has been reported that overexpression of BCL-2 protects cells from damage by radiation and/or chemotherapy, we could not find any significant relationship between the degree of BCL-2 expression and the length of survival of patients with glioblastomas or medulloblastomas.