✓ Intracranial pressure (ICP) was continuously recorded, isotope cisternography was performed, and the ventricular system size was evaluated on serial computerized tomography scans in 39 patients. All of these patients had communicating hydrocephalus after subarachnoid hemorrhage (SAH) from rupture of an intracranial aneurysm. The studies were carried out in both the acute stage (within 7 days after SAH) and the communicating hydrocephalus stage. In patients in the acute stage who had no ventricular dilatation, but who later developed communicating hydrocephalus, the resting ICP was high, and an ICP pattern of B-wave activity was seen; there was no delay in cerebrospinal fluid (CSF) absorption on isotope cisternography. Patients with communicating hydrocephalus in whom ICP recordings were started within 63 days after SAH had a pattern of plateau waves in conjunction with B-waves, and there was a marked delay in CSF circulation. In general, patients with higher resting ICP's had more frequent ICP irregularities. Patients with communicating hydrocephalus in whom recordings were begun more than 6 months after SAH had a low and flat ICP pattern, and there was no delay in CSF absorption in spite of bilateral convexity blocks on isotope cisternography. The results suggest that the ICP pattern of plateau waves in conjunction with B-waves can be regarded as a sign of delayed CSF absorption; hence, shunting procedures may be indicated in patients with plateau waves in conjunction with B-waves visualized on continuous ICP recordings.