Eleven children with acute lymphoblastic leukemia and chronic liver disease, who had negative reactions for hepatitis B virus markers in the liver, were studied at the time of therapy withdrawal for an antigen-antibody system linked to non-A, non-B (NANB) hepatitis infection. By immunofluorescence, six of the 11 children had a positive reaction for the NANB antigen in the liver, and five of these six children also had a positive reaction for the NANB antibody in serum. Histologic lesions were more severe in patients with the NANB antigen in the liver compared with those with a negative reaction.