Fifteen institutions participated in a cooperative study for treatment of single ruptured intracranial aneurysms. Between June, 1963, and February, 1970, 1,005 protocols were submitted to the Central Registry. All patients had a single ruptured aneurysm on the internal carotid, middle cerebral, anterior cerebral-anterior communicating or vertebral-basilar arteries. The four treatments allocated randomly were regulated bed rest, drug-induced hypotension, carotid ligation, and intracranial surgery. This study was an attempt to provide the relative merits of these several modes of therapy.