A 37 year old male developed fever for 20 days, along with headache, anorexia, malaise, sweating, pharyngitis, lymphadenopathy and splenomegaly. At this stage, Ag p24 was positive and anti HIV was negative. The patient recovered fully but 6 months later positive HIV titers were demonstrated by immunofluorescence and Western-blot. A retrospective diagnosis of acute retroviral syndrome was made. The difficult differential diagnosis with infectious mononucleosis, cytomegalovirus, measles, rubella, toxoplasmosis and influenza is discussed. Thus, anti HIV antigenemia should be investigated in any patient with a mononucleosis like syndrome belonging in a high risk group for AIDS, even if Paul-Bunnell-Davidson or IgG anti VCA-EB reactions are positive.