Benedetta Peruzzi,1 Ilaria Cutini,2 Anna Maria Grazia Gelli,1 Tommaso Rondelli,1 Marinella Statello,1 Sara Bencini,2 Francesco Mannelli,2 Roberto Caporale,1 Alberto Bosi,2 Alessandra Fanelli1 1General Laboratory Unit (Microscopy and Clinical Cytometry Unit), 2Hematology Unit, Azienda Ospedaliero–Universitaria Careggi, Firenze, Italy Introduction: Pleural effusion as the first clinical manifestation of acute lymphoblastic leukemia (ALL) is a relatively rare event. An early and accurate diagnosis of this clinical picture is very important for adequate patient management. Case presentation: We report the atypical onset of T-lineage ALL in a 31-year-old man. The patient was admitted to the emergency room due to lung failure; at that moment, the patient's initial blood count was normal; the chest X-ray radiography showed a massive pleural effusion and a thoracentesis was carried out. Routine investigations performed on the pleural fluid using a new technology system and digitalized cell analysis demonstrated infiltration by immature cells. Therefore, bone marrow aspirate and flow cytometry analyses were performed, leading to the diagnosis of T-lineage ALL. A cord blood transplantation procedure was performed at the first hematological remission following chemotherapy regimens. The patient died of septic shock. Conclusion: The case we reported underlines the usefulness of using automated instruments to identify abnormal lymphoid cells in body fluids. Keywords: pleural effusion, digital morphology, leukemia