This paper reviews different methods for measuring iron absorption by using stable or radioisotopes. Methods using stable isotopes are of special interest because of the absence of radioactive exposure. The oldest method is fecal monitoring, which has two drawbacks: the necessity of obtaining a complete collection of the unabsorbed isotopes and sample inhomogeneity. The plasma iron tolerance test seems to be a promising method, although interfering factors should be taken into account, such as gastrointestinal transit time and rate of entry of iron from the gastrointestinal mucosal cell into the bloodstream. The method of choice for measuring iron absorption seems to be the analysis of the incorporation of both an orally and an intravenously administered stable iron isotope into erythrocytes. The only disadvantage of this method is the rather large amount of stable isotopes required. This may be overcome by measuring incorporation of the isotopes into a reticulocyte-rich erythrocyte fraction.