The concentrations of whole blood and plasma selenium, plasma lipid peroxides and the glutathione peroxidase activity in plasma and erythrocytes were measured in untreated and treated children with neuro- (n = 23) and nephroblastoma (n = 32) aged 6 months to 15 years. The patients were divided into the following groups: 0.5-1, 1-3, 3.5-6 and 8-15 years old. In all the groups of cancer patients investigated selenium concentration in whole blood and plasma and glutathione peroxidase activity in erythrocytes and plasma were significantly lower than in the age-matched healthy children. The concentrations of lipid peroxides in plasma of children with cancer was higher than in the age-matched control group. No differences were observed between the levels of the determined parameters in children with neuro- and nephroblastoma. Nor were there any differences in the determined parameters between children before and during treatment with cytostatics and between children at different stages of the disease. A significant linear correlation was found between plasma selenium concentration and glutathione peroxidase activity in the erythrocytes and plasma of children with cancer.