&NA;This is a prospective study of cardiac arrhythmias in patients with acute subarachnoid hemorrhage (SAH) secondary to ruptured aneurysm. Twenty per cent of the patients had serious, life‐threatening arrhythmias. However, 100% of the patients had some kind of cardiac arrhythmia. The arrhythmias occurred during the first 48 hours after SAH. Such arrhythmias occur in patients without overt, pre‐existing heart disease, hypoxemia, or electrolyte imbalance. A prolonged Q‐T interval is frequently observed in patients with SAH who develop serious ventricular arrhythmias. (Neurosurgery,5: 675‐680, 1979)