Antiplatelet drugs are widely used in clinical practice to reduce the risk of adverse cardiovascular events in patients with cardiovascular diseases. It is noted that resistance to antiplatelet agents is determined in a number of patients, which may increase the risk of cardiovascular catastrophes. Given the importance of the antiplatelet component in preventive and therapeutic strategies, the inability to achieve its potential benefit in patients with atherothrombosis is an important clinical problem. Aim of the study. Studying the features of platelet aggregation in vitro in patients with coronary heart disease, acute disorders of cerebral circulation and acute coronary syndrome receiving acetylsalicylic acid or acetylsalicylic acid in combination with clopidogrel during hospital treatment. Materials and methods. The subjects were divided into 3 groups: 1 - patients of the Department of Cardiology - received acetylsalicylic acid - Cardiomagnil ® a dose of 75 mg / day; group 2 - Departments of acute coronary syndrome - took acetylsalicylic acid - Acecardol ® - 100 mg once a day in combination with clopidogrel – 75 mg per day; group 3 - patients of the Department of patients with acute disorders of cerebral circulation took Acecardol ® 100 mg once a day. Platelet aggregation activity was studied in all patients after 5 days of regular intake of antiplatelet agents. Platelet aggregation was studied by optical aggregatometry using a laser aggregometer «Biola». As agonists of aggregation were used: ADP (up to 5 mmol/L), epinephrine (10 mg/ml), collagen (1 mg/ml) and ristocetin (ristomycin) (15 mg/ml). Results and discussion. Insufficient effect on the background of taking antiplatelet agents was observed in 27.2% of cases in patients of group 1 and 8.2% in patients of group 2 (p=0.0034). In patients with acute disorders of cerebral circulation (group 3), the insufficient effect of acetylsalicylic acid was noted in 16.3% of cases, which was significantly less than in group 1 (p=0.026). Patients with acute coronary syndrome receiving double antiplatelet therapy achieved pronounced suppression of aggregation in 54% of cases, which was statistically significantly more than in group 1 - 23.3% (p=0.0004). In patients of group 3, a pronounced antiplatelet effect was observed in 41.2% of cases, which was significantly more than in patients of group 1 (p=0.0054). Thus, the use of acetylsalicylic acid in combination with clopidogrel in patients with acute coronary syndrome provided a more pronounced suppression of platelet aggregation. This is consistent with the data of other researchers who found that in 6% of patients with the use of two antiplatelet agents, the desired result was not obtained [14, 15]. It was noted that in patients with acute cerebrovascular accident who received only acetylsalicylic acid, platelet aggregation decreased to a greater extent than in patients with coronary heart disease (group 1). Perhaps this is due to the fact that patients of these groups received different medications of acetylsalicylic acid. Findings. The use of two antiplatelet agents does not always lead to optimal effect. In this regard, the expediency of aggregatometry to assess the effect of acetylsalicylic acid is obvious. Particular attention should be paid to patients receiving acetylsalicylic acid an outpatient basis, since the low effectiveness of antiplatelet therapy increases the risk of myocardial infarction, stroke.