A Marseillevirus (giant virus of amoeba) has been found in the blood and stool samples of individuals who otherwise appear to be healthy. During an attempt to define a serological cutoff for Marseillevirus by enzyme-linked immunosorbent assay (ELISA) in children, we serendipitously detected high antibody responses to Marseillevirus in an 11-month-old boy suffering from adenitis. Marseillevirus DNA was then found in his blood using PCR and with a unique sequence. We identified Marseillevirus in a lymph node using fluorescence in situ hybridization (FISH) and immunohistochemistry, and the lymph node was removed surgically. The child was declared to be cured 1 year later. We conclude that adenitis during early childhood may be caused by Marseillevirus.