To investigate the cell kinetics of human endometrial disorders immunolocation of PCNA (proliferating cell nuclear antigen) was performed in 69 specimens of normal, hyperplastic, or malignant endometrial tissue that had been fixed in formalin and embedded in paraffin. Immunoreactivity of PCNA was observed in all specimens examined. In the proliferative phase, PCNA positive cells were present in both the glands and stroma. In the secretory phase PCNA positive cells were seen principally in the stromal cells. A PCNA labeling index was obtained by counting one thousand cells per case. PCNA positivity in the proliferative phase was significantly higher than in the secretory phase (P < 0.01), but lower than in moderately differentiated (P < 0.01) or poorly differentiated (P < 0.05) adenocarcinoma. No significant differences in the PCNA labeling index were observed between hyperplasia and adenocarcinoma. These findings indicate that possible biologic differences between these proliferative endometrial lesions are probably not due to differences in cell proliferative activity, but rather to factors other than proliferation such as their ability to invade.