PURPOSE: To determine the utility and tolerability of levetiracetam (LEV) compared to phenytoin (PHT) in preventing clinical seizures in patients with subarachnoid hemorrhage (SAH). METHODS: Utility and tolerability of PHT and LEV in patients with SAH were determined by the occurrence of breakthrough clinical seizures or adverse events necessitating a change of medication. Comparisons were performed with Chi-square tests. RESULTS: All 176 patients were initially treated with PHT. No breakthrough clinical seizures occurred. In 70 (39.8%) patients, PHT was replaced with LEV due to adverse events including elevation of transaminases, thrombocytopenia, unexplained fever, rash, gastrointestinal disturbance, and worsening mental status. After switching to LEV, all adverse effects resolved except gastrointestinal disturbance and worsening mental status in 4 patients. Adverse events occurred more often in patients taking PHT. CONCLUSIONS: In patients with SAH, LEV appears to have superior tolerability compared to PHT.