The objective is to report a case of atypical acute infectious mononucleosis in a juvenile ankylosing spondylitis patient who was treated with infliximab. A 20-year-old man was hospitalized for the evaluation of lymphadenopathy and systemic symptoms. His symptoms developed at the eighth week of the infliximab treatment and he required hospitalization. Lymph node biopsy was performed and he was diagnosed as atypical infectious mononucleosis (absence of fever, pharyngitis, lymphocytosis and negative atypical lymphocytosis on blood smear). Infections have become major concerns in patients treated with TNF-blocking agents. In theoretical base, it is not surprising as TNF-alpha has a crucial role in the body's defense against both bacterial and viral invasion. Blocking the action of TNF may also change the course of the disease and could lead to a delay in the diagnosis. TNF-alpha-blocking treatment may mask the typical symptoms of infectious mononucleosis and atypical cases should be included in the differential diagnosis of lymphadenopathy in patients receiving anti-TNF-alpha agents.