A randomized controlled clinical trial was carried out to study the effect of tranexamic acid (AMCA, Cyklokapron; AB Kabi, Stockholm, Sweden) in the prevention of early rebleeding after the rupture of an intracranial aneurysm. The incidence of vasospasm, hydrocephalus, cerebral ischemic and thromboembolic complications, morbidity, and mortality was also evaluated. The series comprises 59 patients, 30 treated with tranexamic acid and 29 controls. The treatment was stopped if there was rebleeding, operation, or discharge from the hospital. There were 6 recurrent hemorrhages in 6 patients in the tranexamic acid-treated group and 11 recurrences in 7 patients in the control group. Recurrent hemorrhages occurred later in tranexamic acid-treated patients than in controls. Five patients in each group died from rebleeding. Five additional treated patients and 2 controls died from cerebral ischemic dysfunction. The results suggest that tranexamic acid may protect patients with ruptured aneurysms from rebleeding for 1 or 2 weeks, but that it also may produce cerebral ischemic complications.