BACKGROUND Brain Trauma Foundation guidelines recommend seizure prophylaxis for preventing early posttraumatic seizure (PTS). Phenytoin (PHE) is commonly used. Despite a paucity of data in traumatic brain injury, levetiracetam (LEV) has been introduced as a potential replacement, which is more costly but does not require serum monitoring. The purpose of this study was to compare the efficacy of PHE with that of LEV for preventing early PTS. METHODS Consecutive blunt traumatic brain injury patients undergoing seizure prophylaxis were prospectively enrolled at two Level 1 trauma centers during a 33-month period. Seizure prophylaxis was administered according to local protocol. Patients were monitored prospectively throughout their hospital stay for clinical evidence of seizure activity. PHE was compared with LEV with clinical early PTS as the primary outcome measure, defined as a seizure diagnosed clinically, occurring within 7 days of admission. RESULTS A total of 1,191 patients were screened for enrollment, after excluding 378 (31.7%) who did not meet inclusion criteria; 813 (68.3%) were analyzed (406 LEV and 407 PHE). There were no significant differences between LEV and PHE in age (51.7 [21.3] vs. 53.6 [22.5], p = 0.205), male (73.9% vs. 68.8%, p = 0.108), Injury Severity Score (ISS) (20.0 [10.0] vs. 21.0 [10.6], p = 0.175), Marshall score of 3 or greater (18.5% vs. 14.7%, p = 0.153), or craniectomy (8.4% vs. 11.8%, p = 0.106). There was no difference in seizure rate (1.5% vs.1.5%, p = 0.997), adverse drug reactions (7.9% vs. 10.3%, p = 0.227), or mortality (5.4% vs. 3.7%, p = 0.236). CONCLUSIONS In this prospective evaluation of early PTS prophylaxis, LEV did not outperform PHE. Cost and need for serum monitoring should be considered in guiding the choice of prophylactic agent. METHODS Therapeutic study, level III.