We studied the relationships between the blood serum levels of human leukemia differentiation factor HLDF, idiotypic and anti-idiotypic antibodies to HLDF, and clinical indicators of cardiovascular function in apparently healthy individuals and patients with essential hypertension and cerebral hypertensive crisis. Markedly reduced HLDF levels and anti-HLDF antibody titers were found in the blood of the examined patients. Correlations between HLDF levels, duration of hypertension, and systolic and diastolic BP were revealed. These findings suggest that the studied molecular factors are involved in the mechanisms of BP regulation under normal conditions and during hypertension development. The protein HLDF and anti-HLDF antibodies can be considered as biomarkers for early diagnosis of hypertension and its cerebral complications.