Initial cytomegalovirus complement-fixation antibody titers were positive in two of 16 leukemic children (12%) who had received chemotherapy for less than one month. Longitudinal titer determinations in this group showed nine seroconversions, the median time for seroconversion being two months. Illnesses suggestive of viral infection preceded seroconversion in four children. Fulminating, life-threatening pneumonitis developed in one child, who was treated with floxuridine. Antibody titers of initial sera were positive in 12 of 25 children (48%) receiving chemotherapy more than one month. In this second group of children, fluctuations in established titers showed no correlation with clinical events. Further studies are needed to elucidate the pathogenesis of cytomegalic inclusion disease which may be fatal, in transplant recipients and chemotherapy patients experiencing immunosuppression.