Down syndrome (DS) is one of the most common aneuploidy. In general population, its prevalence is 1:600-1:800 live births. It is caused by a trisomy of chromosome 21. DS is phenotypically manifested by premature aging, upward slant to the eyes, epicanthus, flattened face, and poor muscle tone. In addition to physical changes, this syndrome is characterized by early onset of diseases specific to old age, such as Alzheimer's disease, vision and hearing problems, and precocious menopause. Since DS symptoms include premature aging, the shortening of telomeres might be one of the markers of cellular aging. Consequently, the aim of the study was to determine the length of the telomeres in leukocytes from the blood of juvenile patients with DS (n=68) compared to an age-matched control group (n=56) and also to determine the diagnostic or predictive value for this parameter. We show that, for the first time, in juveniles, the average relative telomere length in studied subjects is significantly longer than in the control group (50.46 vs. 40.56, respectively arbitrary units [AU]; p=0.0026). The results provide interesting basis for further research to determine the causes and consequences of telomere maintaining and the dynamics of this process in patients with DS.