Pseudothrombocytopenia is a laboratory artefact that can introduce serious problems in diagnosis and treatment in patients with low platelet count. The most common reason for this artefact is in vitro platelet clumping in blood samples collected into ethilenediaminetetraacetic acid (EDTA) anticoagulant. The clumping activity is greater at temperatures less than 37 degrees C, and the EDTA concentrations required for clumping are 20 times below anticoagulant concentrations. In this article we described the case of a female patient with diagnosed EDTA induced pseudothrombocytopenia. The cause of incorrectly low platelet counts was proved by simultaneous analysis in blood samples collected into EDTA anticoagulant and into heparin as a control sample. Absences of incorrectly low platelet count in heparin sample and rapid decrease of platelet count in EDTA sample were noticed. Decrease in platelet count was accompanied by increase in the number of leukocytes, so called pseudoleukocytosis. Careful examination of blood film is necessary to establish correct diagnosis, promptly after the blood collection and approximately two hours later. It is important to verify formation of clumps two hours after the blood collection and also progressive reduction in the platelet count in EDTA sample. By blood assessment conducted in this concern it is possible to avoid severe misinterpretation in such patients.