Hemostasis consists of complex and interrelated systems whose task is to maintain blood fluidity in the vascular system, while allowing the rapid formation of solid blood clots in the prevention of hemorrhage after a blood vessel injury. The balance between coagulation and fibrinolysis, two key components of hemostasis, has to be strictly controlled. Plasminogen activator inhibitor-1 (PAI-1) is a serpin that prevents the conversion of plasminogen to plasmin by inhibiting its activators - urokinase (uPA) and tissue plasminogen activator (tPA). The PAI-1 gene has a common polymorphism 4G/5G which is located in the promoter region and it affects the activity of PAI-1. People who are homozygotes 4G/4G have a higher plasma PAI-1 activity. Other factors such as age, circadian rhythm, obesity, insulin and glucocorticoids can also influence plasma PAI-1 activity. The aim of this study was to determine the correlation between the 4G/5G polymorphism and the plasma PAI-1 activity in plasma samples of 60 subjects. Additionally, we compared the activities of plasma PAI-1 of female and male subjects. The genotype was determined using the LightCycler 2.0 real-time PCR system (Roche, Switzerland). The activity was determined using the automated hemostasis analyzer Behring Coagulation Timer with the Berichrom PAI test kit (Siemens Medical Solutions, Germany). A trend was found (p=0.092) which confirmed the premise of a higher activity in male patients. A statistically significant difference in the activities between genotypes was not found. This unexpected result can be explained taking into account the small sample size and not considering the other influential factors (age, circadian rhythm, obesity, insulin, glucocorticoids) whilst selecting subject samples.