The bromodeoxyuridine (BrdU) labeling study provides valuable cell kinetic information for individual tumors that could suggest the prognosis of each patient who had a tumor. Recently, a monoclonal antibody against the proliferating cell nuclear antigen (PCNA or cyclin), a nuclear protein expressed in proliferating cells, was developed which could be used on formalin fixed, paraffin embedded tissue. The purpose of this study was to compare the cell kinetic data obtained by the BrdU labeling study and the PCNA method in the same patient. The relationship between labeling indices of BrdU incorporated into S-phase and PCNA expressed by cycling cells was investigated in 31 patients with brain tumors. Both of the labeling indices showed good correlation with histological grade of the tumor. The values of the PCNA labeling index (LI) were parallel but higher than those of the BrdU LI, and the relation PCNA LI = 2.2 x BrdU LI + 0.8 (r² = 0.86) was obtained. The results of this study show that PCNA could replace the BrdU method for identifying the proliferating cells, and the major advantages of PCNA method is that it could be done without any pretreatment and avoid injection of the teratogenic agent for diagnostic purpose.