UNLABELLED Recent evidence demonstrate that masked hypertension (MH) is a significant predictor of cardiovascular disease, while, elevated levels of circulating antibodies against endothelial cell surface antigens (antiendothelial cell antibodies - AECA) seem to play an important role at the early stages of atherosclerosis process and of borderline hypertension as well. Aim of this study was to investigate the presence of AECA in patients (pts) with MH and to compare the AECA title among pts with MH and healthy normotensives (HN), matched for age, sex and body mass index. METHODS One hundred-thirty (60 M, 70 F) healthy subjects mean age 45+/-12 yrs who had clinic blood pressure <140/90 mm Hg were studied. The whole study population underwent 24 hour ambulatory blood pressure monitoring (ABPM). According to the ABPM recordings, 24 individuals (8 M, 16 F) had MH (daytime systolic blood pressure >/=135 mm Hg or daytime diastolic blood pressure >/=85 mm Hg - group A) and the remainder 106 subjects (52 M, 54 F) had normal ABPM recordings, group B. IgG and IgM AECA levels were determined by ELISA method. AECA levels were expressed as mean value+/-SD. None of the study population had a history of connective tissue disease or any metabolic disorder. RESULTS Significantly increased titles of AECA class IgG were found in 8/24 pts of group A (30%) vs. 5/106 (4.6%) of group B (p<0,001). Significantly increased titles of AECA class IgM were also found in 6/24 pts of group A (25%) vs. 3/80 (3.8%) of group B (p<0,001). CONCLUSIONS Our results suggest that patients with MH have significantly higher AECA levels of both classes (IgG, IgM) compared to healthy normotensives. These findings may indicate a possible explanation of the increased cardiovascular risk in MH. The possibility that high AECA levels may be a driving mechanism for the development of MH needs further investigation.