In our Pediatric Haemato-Oncology Unit, 42 young patients cured of their malignancy were left with chronic delta hepatitis. The severity of liver disease in many of these patients prompted us to start a pilot study on the effect of recombinant alpha 2b interferon, given at a dose of 5 MU/square meter thrice weekly. All nine patients included in the study (five males, mean age: 15 years) had well-compensated liver disease, including five cases with active hepatitis and cirrhosis. At the end of the 3rd month of therapy, two patients with cirrhosis developed a biochemical exacerbation leading to hepatic decompensation, which was fatal in one case. The reasons for this unfavourable outcome remain unclear. Basic immunological tests were normal, but one of the two patients was the single case with anti-liver-kidney microsome antibodies. On the other hand, both patients seroconverted from hepatitis B e antigen to antibody at the time of exacerbation, suggesting that liver damage could have been the result of cell-mediated cytotoxicity to hepatitis B virus antigens. The results of this study, which has been interrupted at the 4th month, suggest that interferon therapy for chronic delta hepatitis has to be considered cautiously in young patients cured of pediatric malignancies. In fact, no beneficial effect was seen and the treatment appeared to be harmful in at least two out of nine patients treated.