Tumor repopulation plays a major role in the response of tumors to radiotherapy or chemotherapy but its degree varies widely among human tumors. Its mechanism has not been elucidated, however it is unlikely that repopulation can be simply explained by the greater availability of oxygen and nutrients resulting from the reduction in tumor mass. In normal tissues, such as bone marrow, treated cells release stimulating factors which are able to recruit quiescent cells into proliferation. Similar mechanisms have been observed in experimental tumors and their existence is probable in human tumors. A low pretreatment proliferation index and low cell density indicate that the control mechanisms which regulate cell proliferation in normal tissues are still partially operating in these tumors. In such tumors an increase in the level of cell proliferation during or after treatment has often, but not always, been observed. There is not yet a clear guide for predicting tumor proliferation rate under treatment and further investigations are needed.