The population kinetics of cells within tumors determine both the rates of growth of tumors and their responses to therapy. Of the kinetic parameters that can be measured at present, pre-treatment potential doubling time (Tpot) is thought to be of the greatest value in predicting treatment outcome. Tpot, defined as the time required for a proliferating population to double in number in the absence of spontaneous cell loss, may be measured using flow cytometric techniques following in vivo per-fusion of halogenated pyrimidine analogues of thymidine such as bromodeoxyuridine (BrdUrd). Short Tpot values may identify patients who would be expected to benefit from acceler-ated-fractionation radiotherapy regimens. This paper reviews the literature to date on Tpot. It shows large variations in values even for tumors of similar histologies, indicating a rapid proliferation rate in some tumors. Technical problems in data production are described and ongoing studies discussed.