Numerous studies have shown an association between patent foramen ovale (PFO) and cryptogenic stroke suggesting that paradoxical emboli may be an important cause of stroke. In addition, some authors have proposed that platelet activation is present in PFO patients and this might be the cause of the stroke. The aim of this study was to assess the mean platelet volume (MPV), an indicator of platelet activation and/or reactivity in patients with PFO. The study group consisted of 77 patients with PFO. An age, sex, BMI-matched control group was composed of 43 healthy volunteers. We measured serum MPV values in patients and controls. MPV was significantly higher among PFO patients when compared with control group (9.0 ± 0.8 vs. 8.3 ± 0.9 fl, respectively; P < 0.001). We have shown that MPV was significantly elevated in patients with PFO compared with controls.