Correlation between bromodeoxyuridine-labeling indices and patient prognosis in cerebral astrocytic tumors of adults. Bromodeoxyuridine (BUdR), a nonradioactive thymidine analogue, is taken up by cells in S-phase, and the ratio of BUdR-positive nuclei to the total number of cells counted is defined as the labeling index (LI). In this study, BUdR LI and the clinical course of 50 cerebral astrocytic tumors in adults were analyzed. The obtained LI distributed continuously in a broad range from 0% to 19%. The mean LI of 28 glioblastomas, 12 anaplastic astrocytomas, and ten astrocytomas were 8.5%, 4.2%, and 1.2%, respectively, and these differences were statistically significant (P = 0.05). In the analysis of LI and the recurrence-free period (RFP), regardless of the histologic findings, 23 patients with LI more than 5% had a median RFP of 9.0 months; the median RFP of nine patients with LI of 3% to 5% was 14.7 months. Nine of 13 patients with LI less than 3% have not yet recurred after a median follow-up of 36 months. These differences were also statistically significant by the generalized Wilcoxon test (P = 0.05). The proliferative potential reflected by the BUdR LI is a good clinical indicator for predicting the rate of tumor growth in cerebral astrocytic tumors. In combination with histologic diagnosis, BUdR LI could help in determining a patient's prognosis more precisely. 