Increased aggregation of platelets during preeclampsia was shown in several studies, yet several others reported no change. The aim of our study was to investigate platelet aggregation in a group of patients suffering from preeclampsia. In a cross-sectional study blood samples were taken from 89 hospitalized patients in the third trimester of pregnancy: 38 were suffering from mild to moderate preeclampsia and 51 patients were without preeclampsia. From the blood samples platelet aggregation, secretion of adenine nucleotides from platelets, concentration of energy-rich adenine compounds and levels of cyclic adenosine-mono-phosphate and cyclic guanosine mono-phosphate in platelets were measured. In the patients with preeclampsia, the adenosine diphosphate threshold for biphasic aggregation [odds ratio (OR):.75; 95% Confidence Interval (CI): 0.55–1.02; p<0.05], total adenine nucleotides concentration in the metabolic pool of platelets (OR:0.99; CI: 0.62–1.57; p<0.01) and cyclic adenosine-mono-phosphate (OR:0.81; CI: 0.57, 1.14; p<0.05) and cyclic guanosine mono-phosphate (OR:.78; CI: 0.55–1.09; p<0.05) levels in platelets were decreased in comparison with the control group, while adenylate energy charge in the metabolic pool of platelets (OR: >100.00; CI: 0.00->100.00; p<0.05) and secretion of adenosine triphosphate (OR:.13; CI: 0.00–14.26; p<0.05) and adenosine diphosphate (OR:.77; CI: 0.08–36.79; p<0.05) were increased. The results of our study show increased activation and aggregation of platelets in pregnant females with preeclampsia.