This review summarizes the basic aspects of multidrug resistance (MDR). We first propose a brief reappraisal of the occurrence of MDR encoded P-glycoprotein, (P-g-P) in various normal and tumor tissues. We then present a series of studies undertaken to estimate the MDR1 gene expression or the presence of P-g-P in various human cancers. Finally, we conclude on the potential use of P-g-P measurements in some cancers as a useful if not powerful means of reversing clinical multidrug resistance.