Spontaneous subcapsular splenic hematoma formation without rupture in infectious mononucleosis is a very unusual occurrence. Splenic rupture in infectious mononucleosis (Pfeiffer disease, or glandular fever) is a rare but life-threatening complication. A conservative management is successful in those patients in stable condition. We describe the presentation and the clinical progress of a case in whom a sudden enlargement in hematoma's diameter needed a splenectomy to avoid the risk of blood effusion in the abdominal cavity. The contribution of the sonographic examination and follow-up in the diagnosis of infectious mononucleosis is emphasised.