Proliferating cell nuclear antigen (PCNA), also known as cyclin, is a cell cycle-related nuclear protein that is maximally elevated in late G1 and S phases of proliferating cells. In this study, PCNA was identified by paraffin-section immunohistochemistry in 42 of 64 solid human malignancies, and in several benign tissues known to contain proliferating cells. The PCNA-positive nuclei were randomly distributed and ranged from less than 1% (in most cases) to more than 20% of the neoplastic cells. In general, PCNA positivity correlated with mitotic activity and tumor grade. Further study is necessary to evaluate PCNA as a marker of cellular proliferation and a potential prognostic marker in human malignancy.