The content of malonic dialdehyde (MDA) in erythrocytes and blood plasma as well as the specificity of erythrocyte membrane permeability (EMP) was studied in case of children and teenagers with insulin-dependent diabetes mellitus (IDDM) with respect to a duration and compensation degree of the disease. Eighty-seven children and teenagers with IDDM, aged 7 to 16, and 23 age-matching healthy children and teenagers were examined. The MDA content in erythrocytes and plasma of sick children was found to be increasing yet at the disease onset and did not depend on a duration and compensation degree of diabetes. Erythrocytes of sick children had a lower resistance to the hemolytic action of urea and a higher permeability. As the disease duration was increasing, there was a higher EMP observed at earlier diabetes stages and its decrease with the progressing of diabetes.