We recently showed in an animal model that iron isotopic composition varies substantially between different organs. For instance, iron in ferritin-rich organs-such as the major storage tissues liver, spleen, and bone marrow-contain a larger fraction of the heavy iron isotopes compared with other tissues, including blood. As a consequence, partitioning of body iron into red blood cells and storage compartments should be reflected in the isotopic pattern of blood iron. To confirm this hypothesis, we monitored blood iron isotope patterns in iron-overloaded subjects undergoing phlebotomy treatment by multicollector inductively coupled plasma mass spectrometry. We found that bloodletting and consequential replacement of lost blood iron by storage iron led to a substantial increase of the heavy isotope fraction in the blood. The progress of iron depletion therapy and blood loss was quantitatively traceable by isotopic shifts of as much as +1‰ in δ((56)Fe). These results show that-together with iron absorption efficiency-partitioning of iron between blood and iron storage tissues is an important determinant of blood iron isotopic patterns, which could make blood iron isotopic composition the first composite measure of iron metabolism in humans.