A newly developed in vitro labeling method with bromodeoxyuridine (BrdU) identifies S phase cells in situ in freshly obtained surgical tissue of human brain tumors which is subsequently fixed and embedded in paraffin for BrdU immunovisualization. For the first time, the BrdU labeling index (LI) is successfully compared here with the LI obtained by immunostaining of frozen sections of the same tumors with monoclonal antibody Ki-67 which identifies all proliferating cells, i.e., the growth fraction. LIs were counted in at least five different areas with high density of labeled cells; at least 1,000 cells were counted. In 13 metastatic tumors, Ki-67 LI was 8.3%-62.6%, and BrdU LI was 5.1%-28.0%. In 18 gliomas, Ki-67 LI was 1.4%-19.3%, and BrdU LI was 0.2%-11.6%. In 7 meningiomas, Ki-67 LI was 0.3%-3.0%, and BrdU LI was 0%-2.0%. Statistical comparison of Ki-67 and BrdU LIs by linear regression analysis revealed a highly significant correlation: BrdU LI = 0.99 + 0.34 Ki-67 LI (r = 0.92, P less than 0.001). A significant heterogeneity of proliferation patterns may occur within one sample from area to area, as well as between different samples of the same tumor, especially in gliomas; thus, some subjective influence on LIs by arbitrary sampling and selection could occur in quantitative evaluation of in situ cell kinetics of human brain tumors. This study indicates that our in vitro BrdU-labeling method allows the in situ identification of S phase cells in excellently preserved fixed tumor tissue which is well suited for further histological examination.(ABSTRACT TRUNCATED AT 250 WORDS)