The acute hydrocephalus in patients with nonaneurysmal perimesencephalic hemorrhage suggests an extraventricular obstruction of CSF flow. We studied the occurrence of acute hydrocephalus and the site of cisternal blood in 40 consecutive patients with perimesencephalic hemorrhage. In all 11 patients with hydrocephalus, all perimesencephalic cisterns were filled with blood; this occurred in only five of the 29 patients (17%) without hydrocephalus (p less than 0.0001). We conclude that in the absence of intraventricular blood, filling of all perimesencephalic cisterns with blood is a necessary factor for the development of acute hydrocephalus.