Four males and two females, aged 13 to 57 years (median 22 years), with acquired severe aplastic anemia (AA) were treated with intravenous bolus of high doses of 6-methylprednisolone (MPL). Patients received MPL within a 30-day period at a dose of 20 mg/kg/day (3 days), 10 mg/kg/day (4 days), 5 mg/kg/day (4 days), 2 mg/kg/day (9 days), and 1 mg/kg/day (10 days). Within the first 3 months following MPL therapy, a response rate of 83%, assessed by means of increase in reticulocytes, neutrophils or platelets, was recorded in the group: two cases showed partial response and three improvement. The 3-month, and 1-, 2- and 3-year survival of the group was 67%, 50%, 33% and 33%, respectively. Neither the presence of reticulocytopenia or thrombocytopenia prior MPL therapy, nor age, gender, etiology of AA or time between diagnosis and initiation of MPL influenced survival. In contrast, neutrophil counts before MPL treatment had a strong prognostic value. Patients with < 0.5 x 10(9)/L neutrophils had a median survival of 4.2 months (range 1.2 to 5.2 months) as compared to the 36.1 months median survival (range 12.1 to 36.8 months) of patients whose neutrophil counts were > 0.5 x 10(9)/L. Follow-up data suggest that the administration of androgens two months after MPL therapy did not modify survival. It is concluded that high-dose MPL is useful in the treatment of some patients with acquired severe AA, particularly in those with > 0.5 x 10(9)/L neutrophils who are not candidates for bone marrow transplantation.