We investigated the effect of human recombinant DNA-derived IFN-alpha-2 given in a dose of 1-2 X 10(6) units daily by subcutaneous injection to five patients with advanced idiopathic myelofibrosis (IM). Transfusion dependent anemia and symptomatic splenomegaly were taken as inclusion criteria for this pilot study. Two patients succumbed, one and three months after starting interferon-treatment because of pneumonia and traumatic cranial injury, respectively. While on IFN-treatment no improvement of cytopenia or reduction of splenomegaly was seen in four of the patients. In one patient, however, the requirement for erythrocyte transfusions decreased from 5 to 1.7 monthly upon IFN-treatment. After two, four and six months respectively IFN-treatment had to be stopped in these cases because of progressive thrombocytopenia and/or neutropenia. These observations suggest, that IFN-alpha might be of only marginal value in the treatment of advanced idiopathic myelofibrosis.