A group of 90 patients with acute lymphoblastic leukaemia (ALL) in first continuous complete remission (CCR), admitted in our hospital between January 1986 and September 1992, were tested for the presence of antibodies against hepatitis C virus (HCV), antibodies against hepatitis B virus and antibodies against HIV-1 during maintenance therapy or thereafter. They were compared with a group of 71 children with other malignancies in first CCR who had been diagnosed consecutively from January 1986 to September 1992. No patient with ALL or any other malignancy was found to be positive for hepatitis B surface antigen or HIV-1. HCV-specific antibodies were detected in 28 out of 87 children (32.1%) with ALL and in 4 out of 44 patients (9%) with malignancies other than ALL who had received at least one transfusion of blood or platelets (P < 0.01). HCV-specific antibodies were also detected in one out of three untransfused children with ALL but in none of the untransfused children with malignancies other than ALL. HCV-specific seropositivity influenced the management of children with ALL during maintenance therapy. In fact, as a result of abnormal liver function tests, maintenance therapy had to be suspended significantly more often in the case of HCV-seropositive patients with ALL than in HCV-seronegative ones. Despite the high morbidity during maintenance therapy, chronic liver disease (CLD) was uncommon in both groups: five children with ALL (17.2% of HCV-seropositive children) and one child with a malignancy other than ALL (25%) had CLD.(ABSTRACT TRUNCATED AT 250 WORDS)