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Elevated levels of soluble cellular adhesion molecules have been reported in patients with acute coronary syndromes. Likewise, a relation between decreased nocturnal melatonin levels and coronary artery disease has been suggested. The aim of the present study was to investigate the day–night variations in the concentration of soluble vascular cell adhesion molecule-1 (sVCAM-1) in patients with ST-segment elevation myocardial infarction (STEMI) in relation to the light/dark melatonin pattern. Ninety consecutive patients with STEMI who were admitted to the Coronary Care Unit of our institution were studied. We also recruited 70 age- and gender-matched healthy normal subjects. Blood samples were drawn at 09:00 and 02:00 hr, while patients were at rest, for the assessment of sVCAM-1 and melatonin, which were measured using commercially available ELISA. In STEMI patients, melatonin concentrations maintained a diurnal variation, but the difference between nocturnal and diurnal levels was less than that in healthy subjects ( P < 0.0001). In contrast to findings with melatonin, sVCAM-1 levels showed no diurnal variations in control subjects. In the STEMI group, however, sVCAM-1 concentration at 02:00 hr was significantly higher than that during the light phase (09:00 hr; 1391 ± 38 versus 1200 ± 43 ng/mL, P < 0.05). The results suggest that diurnal variations in endogenous sVCAM-1 production in STEMI patients might be related to an attenuated circadian secretion of melatonin. [ABSTRACT FROM AUTHOR]