Cell proliferation was evaluated by Ki67 monoclonal antibody in 33 colorectal adenocarcinomas and in the normal colonic mucosa. Immunoreactivity was assessed independently by two observers in two subsequent evaluations with a semiquantitative method, by counting at least 2000 cells in two distinct neoplastic specimens (central and peripheric section). There was an excellent intra-inter observer agreement in Ki67 score for each specimen. The tumor score range from 7 to 70% (median 48.8), without any significant correlation with sex and age of the patient and location, size, staging and grading of the neoplasm. Tumor Ki67 score was almost identical in central (46.96%) and in peripheral section (49.24%), and always higher than in normal mucosa. There was no distinction in Ki67 score in normal mucosa at various distances from the tumor. In our experience, Ki67 provides a reliable and reproducible method for assessment of proliferative activity; its clinical applications need further studies.