Leukemia inhibitory factor (LIF) exerts multidirectional effects in the setting of essential hypertension (EH). There is a mounting body of evidence refuting the postulate about identical STAT3 signaling in cardiomyocytes and endothelial/smooth muscle cells, which is important in the situation of extended exposure to gp 130 ligands (LIF in particular). At the same time, there are no reports on the circadian dynamics of peripheral blood LIF concentrations and possible secondary changes to the pathophysiological effects of this cytokine. This study aimed to analyze the circadian dynamics of LIF concentrations in the peripheral blood serum measured at 5 different time points in patients with stage II EH in the presence/absence of antihypertensive therapy and their relationship with the frequency of complications developing within a 5-year follow-up. Blood serum LIF was measured in 60 patients with stage II EH using ELISA at 8:00, 14:00, 20:00, 2:00, and 8:00 o’clock before putting the patients on antihypertensive therapy and one year after its onset. The identified patterns of diurnal LIF concentrations (a rise by ≥15% at 20:00, p < 0.001; a further rise by ≥22% peaking at 2:00, p < 0.001 relative to the values at 8:00) can be regarded as pathologic; their persistence after one year of antihypertensive therapy is a sign of EH progression and puts the patients at 6-fold risk for cardiovascular complications, including myocardial infarction and acute cerebrovascular events.