We investigated the case of a 14-year-old girl with an ethylenediaminetetraacetic acid (EDTA)-dependent haemagglutination detected by macrocytosis, which was only evident by an abnormal red blood cell (RBC) population in the histogram. Investigations included haemograms with different anticoagulants and experimental conditions. Immunohaematological studies were performed using a gel-based technology. At admission, the patient had a low RBC count and an increased mean corpuscular volume with normal haemoglobin. A double population appeared in the RBC histogram. However, the peripheral blood smear was normal and macrocytosis was absent when heparin or citrate was used instead of EDTA. Later studies revealed that the patient's serum was able to induce macrocytosis of control RBC only in the presence of EDTA. An EDTA-dependent panagglutinin was then indentified that produced mixed field agglutination. These findings provide evidence of a haemagglutination induced by EDTA as a source of pseudomacrocytosis.