Erythrocyte deformability is important property of erythrocytes that considerably affects blood flow and hemodynamics in general. The high content of polyphenols present in dark chocolate has been reported to play a protective role in functionality of erythrocytes. We hypothesized that chocolate might influence erythrocytes not only after repeated chronic intake, but also immediately after its ingestion. Thus, we determined the acute effect of dark chocolate and milk (with lower content of biologically active substances) chocolate intake on erythrocyte deformability. We were also focused on selected factors that may affect erythrocyte deformability, specifically nitric oxide (NO) production in erythrocytes and total antioxidant capacity of plasma. We determined post-treatment changes in mentioned parameters 2 hours after consumption of chocolate compared with their levels before consumption of chocolate. Erythrocyte deformability was determined using filtration method and NO production in erythrocytes using fluorescent NO probe DAF-2 DA. The total antioxidant capacity of plasma was measured by determining the trolox equivalent antioxidant capacity. The study was approved by the Ethics Committee of Faculty of Medicine, Comenius University and University Hospital in Bratislava, Slovak Republic. Written informed consent was obtained from all the subjects. The study conformed to the code of ethics stated in the Declaration of Helsinki. In contrary to milk chocolate intake, the dark chocolate led to significantly higher increase in erythrocyte deformability (3.5±0.7% dark vs. 0.4±1.1% milk, p=0.016). NO production in erythrocytes was not changed after dark chocolate intake (6437±344 before vs. 6353±385 after, p=0.77,) but significantly decreased after milk chocolate (6430±436 after vs. 7426±636 before, p=0.003). The plasma total antioxidant capacity remained unaffected after ingestion of both chocolates. We may conclude that our hypothesis was confirmed. Single ingestion of dark chocolate improved erythrocyte deformability despite unchanged NO production and antioxidant capacity of plasma. Increased deformability of erythrocytes may considerably improve rheological properties of blood and thus hemodynamics in humans, resulting in better tissue oxygenation.