The relationship between food iron absorption, iron stores, and plasma iron level was studied. On a low iron diet subjects with idiopathic hemochromatosis (IH) during reaccumulation of iron after phlebotomies showed a fall in plasma iron. Fortification of the diet with 22–135 mg of iron/day for 3 days caused little or no change in the plasma iron in subjects with normal iron stores, whereas in subjects with iron deficiency a significant rise in plasma iron occurred with the addition of 45 mg of iron/day. In subjects with IH with normal iron stores, plasma iron increased with the addition of 22.5 mg/day. These studies indicate that iron absorption is an important determinant of the elevated plasma iron in IH and that the plasma iron tolerance test combined with the serum ferritin may be used to detect excessive absorption of iron.