The induction of benign prostatic hyperplasia (BPH) from normal prostate is obviously associated with a distinct increase in epithelial and stromal proliferation. We have shown previously that the further increase of BPH volume in aging men is not associated with a further increase in proliferation. We studied whether an imbalance between programmed cell death (apoptosis) and cell proliferation may explain continuing growth in aging men. In prostates of 17 men with BPH removed by open prostatectomy proliferating cells were localized immunohistochemically with the Ki-67 antibody. Apoptotic cells were detected with the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) method. Proliferation and apoptotic index was calculated with a computer assisted image analysis system. Mean proliferation index +/- standard deviation in epithelium (0.142 +/- 0.097) and stroma (0.121 +/- 0.082) was nearly identical. Mean apoptotic index in epithelium (0.172 +/- 0.156) was negligibly higher than the corresponding proliferation index. In stroma, however, no apoptotic cells were detectable. Proliferation index and apoptotic index in epithelium and proliferation index in stroma showed no correlation to patient age or prostate volume. In the epithelium of BPH, obviously the cell kinetic is balanced. On the other hand, our results indicate stromal growth due to cell proliferation in the absence of cell death. This may explain the continuous increase of BPH volume in aging men.