Packed red blood cell (PRBC) transfusions are an important part of supportive treatment in oncology; however, when used frequently, they can be a result of transfusion-related iron overload. The aim of the study was to evaluate the role of ferritin as a non-specific marker of neoplastic growth and transfusion-related iron overload in children with lymphomas and solid tumors. We performed a longitudinal analysis of PRBC transfusions and changes in ferritin concentrations during oncological treatment of 88 children with lymphomas and solid tumors. A ferritin concentration above 500 ng/mL was diagnosed in 14.77% of patients at the moment of admission and 18.18% at the end of treatment. No differences were shown in serum ferritin in the context of tumor type-, sex-, and transfusion-related parameters. Those above the age of 10 demonstrated higher ferritin concentrations compared to subjects younger than 5 years of age. In addition, those over than 10 years old or above 30 kg in weight showed a tendency for better survival. All tested patients demonstrated highly significant correlations between ferritin at the 15th month of treatment or after therapy discontinuation and transfusion-related parameters. Interestingly, ferritin levels were found to lower back to the values before therapy shortly after its discontinuation. Transfusion parameters and ferritin levels had no influence on the survival of the studied cancer patients.