Background: In humans approx. 10% of the total body selenium (Se) content is present in the blood being evenly distributed among plasma and red cells. The important role of Se in antioxidative biological pathways is proven. Many parents of children with malignancies ask for supplementation with Se as part of complementary therapy during or after the oncological treatment. However, toxic Se concentrations may easily be reached in children. In order to analyse whether Se is also supplied by red cell transfusions (RCT), we determined Se concentration in whole blood prior and after packed RCT in pediatric patients with hemato-oncological diseases. Patients and Methods: EDTA-blood was collected from 17 patients (median age: 4 years, range: 1 month - 17 years) with aplastic anemia, acute leukemia and solid tumours prior and after RCT (n = 60). Patients received a median of 2 transfusions (range: 1-14). Samples were also collected from the transfusion blood bags and Se concentration was determined quantitatively by atomic absorption spectrometry. Results: 95% of the specimen collected from the transfusion bags exhibited selenium concentrations within the normal adult range. Mean Se concentration in the patients' blood prior to RCT was 66.2 μg/l (range: 38.0-166.4 μg/l) and increased to 70.7 μg/l (range: 14.1-105.1 μg/l) thereafter (statistically not significant). Applying age dependant reference values Se concentrations were below the lower limit in 45% of the samples prior to RCT and only in 26% after RCT. The reason for this increase was the fact that Se concentrations were often just marginally below the age-dependant lower limit prior to RCT and in the lower normal range thereafter. Conclusion: 43 % of the patients with hematooncological diseases in this study exhibited no Se deficiency at any time point. In the remaining 57% of the patients a transient or persistent Se deficiency was detected with blood levels partially far below the lower threshold of the age adjusted normal range. The Se deficiency was corrected in four out of eight patients by RCT. As Se levels may fluctuate in individual pts a supplementation should only be initiated if based on regular monitoring of the Se concentration.