Since 1977 the bioavailability of 14 bi- and trivalent oral iron preparations has been investigated in five separate orientated clinical studies by using postabsorption serum iron concentration curves. The range of relative bioavailability was 46 to 100% in the first group of preparations, 31 to 47% in the second group and 6 to 29% in the third group. The first group contained mainly bivalent quick release preparations, the second group slow or sustained release preparations and the third mainly trivalent iron preparations. The postabsorption serum iron concentration curves which show a good congruence with exact 59Fe whole body retention tests again confirmed the nearly 50-year-old rule that bivalent iron is up to 16 times better absorbed than trivalent iron. There is no doubt that the oral iron preparations of good bioavailability are able to cure an iron deficiency more rapidly than iron preparations with a low bioavailability. This therefore has a clear influence on the overall expense of iron therapy. Only those preparations from the first group can be recommended for oral iron therapy. The preparations in the second group are less suitable and those in the third group should be excluded from iron therapy in all countries.