In order to define new prognostic factors useful for therapeutic decision-making, the Authors conducted a study on anal canal carcinomas in which Ki-67 proliferation index is correlated with pathological variables and clinical outcome. The Ki-67-detectable antigen is expressed in all stages of the cells cycle except G0. Thus, Ki-67 index can measure cell proliferation and it could be considered an indicator of prognosis. Thirty-one patients with anal canal carcinoma were evaluated. The specimens were formalin-fixed, paraffin-embedded and used for immunostaining of Ki-67 antigen. We found a significant correlation between Ki-67 score and depth of invasion and lymph node involvement. No correlation was found between high Ki-67 value and neoplastic relapse. These results suggest that Ki-67 positivity carries different significance in different cancers. Additional studies are required to ascertain whether more aggressive therapeutic procedures should be applied in the subset of patients with a high growth fraction.