Expression and location of Proliferating Cell Nuclear Antigen in epithelial nuclei were assessed in invasive adenocarcinoma of the prostate gland. The PCNA-positive nuclei showed homogeneous or granular types of immunostaining or a mixture of both, and a gradation in the intensity of staining. Nuclei with homogeneous pattern appeared darker brown than the lighter granular and mixed patterns. Darker nuclei were quite frequently noted, mainly among the epithelial cells adjacent to the stroma. For the marginal zone of invasive adenocarcinoma, the mean proportion of PCNA-stained nuclei in the small acinar pattern was somewhat similar to that in the large acinar pattern, i.e., 8.66% and 9.06%, respectively. In contrast, the mean values in the cribriform pattern were greater than in the small and large acinar patterns, and decreased from the nuclei in the basal position, or adjacent to the stroma, toward the lumen: 14.40% in the basal position, 11.84% in the intermediate and 9.26% in the lumenal. In the solid/trabecular pattern, the proportions of PCNA-positive nuclei were higher than in all the other patterns: 17.60% in the cell layer adjacent to the stroma and 13.88% in the other layers. The trend of value changes in the central zone of the tumour was similar to that obtained in the marginal zone. However, the proportions were lower and the differences statistically significant. This might indicate that the proliferation state is higher in the marginal zone and that the tumour grows eccentrically rather than centrally.