Abstract Objective: Being the earliest step on the way to atherosclerosis, endothelial dysfunction is particularly escalated in diabetes. This study aimed at assessing endothelial dysfunction and blood pressure disturbances in young patients with type 1 diabetes mellitus (T1DM) and defining their interrelations. Methods: The study group comprised 52 children and adolescents aged 14.07 ± 3.03 years, with T1DM duration 5.13 ± 2.18 years. 20 healthy controls with similar age and sex distribution were included. Chosen serum biochemical markers of endothelial damage: intercellular adhesion molecule-1 (sICAM-1), vascular cell adhesion molecule-1 (sVCAM-1), sE-selectin, tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6) as well as ambulatory blood pressure monitoring (ABPM) were performed in all subjects. Results: Patients with T1DM displayed significantly higher concentrations of chosen markers of endothelial dysfunction compared to controls (sVCAM-1 (ng/ml): 951.56 ± 330.68 vs. 710.35 ± 162.12, TNF-α (pg/ml): 16.63 ± 8.32 vs. 9.41 ± 4.23, IL-6 (pg/ml): 3.38 ± 1.31 vs. 2.45 ± 0.81; p < 0.05). Within the study group subjects with an abnormal ABPM reading had significantly higher concentrations of sE-selectin compared with subjects with normal ABPM (in ng/ml: 45.71 ± 15.63 vs. 32.42 ± 11.95; p < 0.01). The study revealed a significant positive correlation between sE-selectin and systolic as well as diastolic pressure loads during the day period (respectively: r = 0.46, r = 0.60; p < 0.01). Conclusions: Endothelium dysfunction may be present early in the course of T1DM in children and adolescents. It seems to be related with blood pressure disturbances which highlights the need to intensify treatment in this group of patients.