There are few studies on the proliferative activity of cells in Kaposi's sarcoma. It remains unclear whether the different progressive histologic stages of Kaposi's sarcoma correlate with the proliferative activity of the tumor. To clarify this issue, we studied the Ki-67 proliferation index in 40 specimens of acquired immune deficiency syndrome-related Kaposi's sarcoma in correlation with the histologic stage of the lesions. The mean proliferation index in our combined material was rather low (8%, range 1-20%) and there was no significant difference among the different stages of the disease. Contrary to a common and probably misleading concept, our results suggest that the histologic progression of the Kaposi's sarcoma is not related to an increase of the proliferative compartment.