The absorption of iron given per os to ten polycythemic patients was examined by two methods: first by giving radioactive iron (59Fe) and utilizing a whole-body counter, and second by peroral administration of ferrous sulfate. A corelation of 80% was achieved by the two methods. A comparison was made between the patterns of the ferrous sulfate absorption in the polycythemic patients and in five patients with low iron serum due to bleeding hemorrhoids. No difference was observed between these two. groups, with the exception of the initial values which were lower in the polycythemic patients. On the other hand, the absorption of iron was higher in polycythemic patients, indicating that serum iron level is not the only factor regulating iron absorption.