One hundred twenty-eight serum samples, collected from infants transfused at the neonatal care unit of Kyushu University Hospital, Fukuoka, Japan, were tested for the presence of antibody to human T-cell lymphotropic virus type I (HTLV-I) by the enzyme-linked immunosorbent assay and the indirect immunofluorescence test. Three of 128 were positive for anti-HTLV-I antibody. The three seropositive patients had received 62, 82, and 1160 mL of transfused blood. These patients did not receive any transfusion thereafter. In addition, no anti-HTLV-I antibodies were detected in the sera of their mothers and siblings. These data suggested that HTLV-I infection is one of the transfusion-associated complications in neonates.