After organ transplantation, life-long immunosuppression is mandatory to prevent rejection. This is not the case after allogeneic bone marrow transplantation (BMT). The mechanisms of tolerance are little understood and there is little data about the required duration of immunosuppression. In a retrospective study we analyzed the use of cyclosporine A (CyA) treatment in all BMT patients who where transplanted in Basel from 1979 to 1991. 243 patients with a median age of 26 (2-49) years received CyA to prevent graft-versus-host-disease (GvHD) and rejection. 206 patients had leukemia, 26 severe aplastic anemia and 10 a lymphoproliferative disorder. All were treated according to a common CyA regimen with initial parenteral administration followed by oral treatment for a year; after 1988 treatment duration was reduced to 6 months. Drug dosage was adjusted according to clinical symptoms, plasma levels and toxicity. At relapse of the original disease CyA was always withdrawn. Three months after BMT 96% were still on CyA. After 6 months 79%, after 1 year 61%, after 2 years 29%, after 3 years 18%, after 4 years 15%, after 5 years 8% and after 8 years 2% of patients were still taking CyA. All 20 patients who are still alive 10 years after BMT are off CyA. CyA was administered parenterally for a median of 31 (5-147) days. Oral treatment started at day 25 (4-28). The oral dose was reduced 5 (1-20) times, increased once (1-75) and finally discontinued after a median time lapse of 1 year (14d-8y).(ABSTRACT TRUNCATED AT 250 WORDS)