Emergence of iron aggressive forms in patients with hematological or oncohematological diseases is potentially life-threatening. This can be caused by several situations: multiple allogeneic RBC transfusions, increased intestinal absorption of iron, or damaging effects of chemotherapeutic agents. The objective of the study was to identify candidates for iron-chelating therapy. The data on 727 patients with screening blood tests were analyzed. The highest serum ferritin level was revealed in patients with thalassemia, PMF, or MDS. They received 80, 37, or 35 RBC transfusions, respectively. The lowest serum ferritin levels were found in patients with CLL or hemolytic anemia that received lesser number of RBC transfusions, namely, 18.5 and 16.5, respectively. The correlation between serum ferritin level and the total number of RBC transfusions was revealed: r = 0.462; p = 0.000. We concluded that iron-chelating therapy is primarily indicated to the patients with MDS or PMF in whom the high serum ferritin level is due to excessive post-transfusion iron.