Patients with sleep apnoea have a significant alteration in the day-night pattern of myocardial infarction and sudden cardiac death observed in the general population. The aim of this study was to investigate the influence of sleep apnoea on the diurnal variations in various haemostatic parameters (factor VII, von Willebrand factor and plasminogen activator inhibitor (PAI)-1) and markers of endothelial dysfunction (asymmetric dimethylarginine (ADMA) and soluble CD40 ligand (sCD40L)). We studied 26 male patients with obstructive sleep apnoea syndrome (OSAS; 13 patients with severe OSAS (apnoea/hypopnoea index (AHI) >30 events · h(-1)) and 13 patients with mild-to-moderate OSAS (AHI <30 events · h(-1))) and 12 controls of similar body mass index (BMI) and waist circumference. In each subject, six different samples were obtained over 24 h. Although all the markers values tended to be higher in patients with severe OSAS, differences did not reach statistical significance at any time. PAI-1 levels were significantly related to BMI (p<0.001), mean (p<0.001) and minimal (p = 0.047) nocturnal oxygenation saturation. ADMA levels were significantly related to arousal index (p = 0.046). The results of this study suggest that day-night variations in factor VII:antigen (Ag), von Willebrand factor:Ag, PAI-1, sCD40L and ADMA levels may be dependent on either the obesity index or metabolic dysfunction rather than on sleep apnoea alone.