A child undergoing induction therapy for acute lymphoblastic leukemia suffered an aplastic crisis associated with B19 virus infection. Good response to the antiblastic therapy led to a burst in erythropoiesis favoring high viral replication, which was responsible for strong erythroblastic inhibition and severe viremia. The patient's B19 antibody response became evident very late, probably because of the antiblastic effect of the therapy; nevertheless, recovery was complete in little more than a week, favored by B19 IgG transfusion with a red blood cell concentrate. This report suggests that immunosuppressed subjects, as well as those suffering from hemolytic anemia must also be considered "at risk" for aplastic crisis due to B19 infection.