Selenium (Se) state was investigated in 86 children (age range 0.9 - 20 years) with solid tumors: neuroblastoma, germinal tumors or osteosarcoma at different therapeutical phases. NB 90 P, MAKEI BEP 89 and COSS 90 therapy protocols were used according to the German Society of Paediatric Oncology and Haematology. Results of treatment cycles were compared to each other and to the Se state of healthy children of the same age and sex. Whole blood and plasma Se was determined by AAS with hydride generation, simultaneously glutathione peroxidases (GSH-Px) in erythrocytes and plasma were measured. At the time of diagnosis, prior to chemotherapy, surgical or any other intervention (group A n=15) whole blood Se (0.8±0.12 and 0.75±0.16 μmol/L versus 1.05±0.14 and 1.12±0.25μmol/L) and plasma Se 0.56±0.05 and 0.64±0.15 μmol/L versus 0.83±0.11 and 0.93±0.20 μmol/L) were significantly lower in tumor patients under or above the age of 7 years. GSH-P% activities in plasma of young children and teenagers 76±18 and 93±13 versus 105±20 and 126±31) and in red blood cells of teenagers(5.4±1.1 vs 6.8±1.7) were significantly lower in newly diagnosed tumor patients compared to healthy children. The lowest Se status was observed in the youngest neuroblastoma toddlers. During intensive chemotherapy (group B1 n=24) selenium concentrations and glutathione peroxidase activities were the lowest observed irrespective of the type of tumor. Under continuous maintenance chemotherapy (group B n=25) or therapeutic intervals (group C n=10) Se concentrations increased by 30-40%. GSH-Px enzyme activities also recovered compared to group A. In patients who did not need therapy (group D, n=20) for at least one year. Se concentrations were twice as high as in newly diagnosed tumor patients (group A). Their Se state was as high as in healthy children of the same age. No Se supplementation was applied in the tumor patients.