The prognosis of patients with severe capillary leakage syndrome (CLS) after bone marrow transplantation (BMT) is dismal despite aggressive use of intensive care therapy. Because the activated classical pathway of complement and relatively low levels of C1 esterase inhibitor (C1 INH) activity are known features in these patients, we evaluated the efficacy of a therapy using purified, human C1 INH concentrate. Severe CLS was defined as increase in body weight by more than 3% within 24 h combined with generalized edema, impaired hemodynamic system (tachycardia and/or decreased blood pressure), and non-responsiveness to furosemide. Of 142 patients, 22 developed severe CLS. The first seven patients whom we diagnosed with this complication were assessed as control patients. Fifteen patients with severe CLS were treated with C1 INH concentrate using a cumulative dose of 180 units/kg body wt. (initial dose: 60 units/kg, followed by two doses at 30 units/kg and four doses at 15 units/kg, every 12 h). The survival rate of patients with CLS was 57% at 1 year after BMT in the C1 INH treatment group, compared with 14% in the control group (p = 0.008). Eight of 15 treated patients are alive at a median of 9 months (range: 4-55) after BMT. The plasma levels of the complement activation parameters C4d and C5a were 3 +/- 1.1 mg/dl (mean +/- S.D.) and 0.3 +/- 0.1 microgram/l, respectively, prior to BMT, increasing to 8.2 +/- 2.1 mg/dl and 1.3 +/- 0.4 micrograms/l, respectively, at diagnosis of CLS. After infusion of C1 INH concentrate the plasma levels of C5a and C4d normalized. The activity of C1 INH rose to 139 +/- 10% of normal human plasma NHP pool (mean +/- S.D.) after infusion. The CH50 values were not significantly altered. The fluid status normalized within 11 days in 14 of 15 treated patients. The results of this study suggest that therapy with C1 INH concentrate improves the prognosis of patients with CLS after BMT. This has to be confirmed in a randomized, controlled trial.