D-dimer, a degradation product of fibrin, is being increasingly used as a marker and prognostic factor in various thrombotic diseases. Previous reports have shown that obstructive sleep apnea is associated with platelet activation and hypercoagulability. The aim of the study was to assess the potential role of the plasma D-dimer test in patients with obstructive sleep apnea. We designed a prospective group comparison study in a tertiary-care, university-affiliated medical center. One hundred and three patients of mean age 57 years (range 50-76 years) with symptoms suggestive of obstructive sleep apnea were included. Polysomnography was performed in all cases, and blood was collected for plasma D-dimer measurement by MiniQuant turbidmetric assay. The demographic and polysomnograph data were compared between patients with normal and high (> 250 ng/ml) D-dimer levels. The group with higher D-dimer values had lower mean minimal oxygen saturation (72.1 +/- 16.4 vs. 81.7 +/- 11.6%, p = 0.008) and a longer mean period of oxygen saturation below 90% (84.1 +/- 86.2 vs. 38.5 +/- 70.8 minutes, p = 0.032). There was no correlation of respiratory disturbance index and sleep architecture with D-dimer values. We concluded that sleep apnea syndrome is associated with fibrinolytic activity. Oxygen desaturation seems to be one of the mediatory factors in the putative connection between obstructive sleep apnea and hypercoagulability state.