We aimed to elucidate platelet function in trauma patients, as it is pivotal for hemostasis yet remains scarcely investigated in this population. We conducted a prospective observational study of platelet aggregation capacity in 213 adult trauma patients on admission to an emergency department (ED). Inclusion criteria were trauma team activation and arterial cannula insertion on arrival. Blood samples were analyzed by multiple electrode aggregometry initiated by thrombin receptor agonist peptide 6 (TRAP) or collagen using a Multiplate device. Blood was sampled median 65 min after injury; median injury severity score (ISS) was 17; 14 (7%) patients received 10 or more units of red blood cells in the ED (massive transfusion); 24 (11%) patients died within 28 days of trauma: 17 due to cerebral injuries, four due to exsanguination, and three from other causes. No significant association was found between aggregation response and ISS. Higher TRAP values were associated with death due to cerebral injuries (P < 0.01, when corrected for ISS and platelet counts), whereas lower platelet counts were associated with massive transfusion (P < 0.01, when corrected for ISS and aggregation). An aggregation value of 145 IU by TRAP significantly identified death due to cerebral injury (sensitivity 71% and specificity 76%, P < 0.01) by receiver operating characteristic-curve analysis; the corresponding value of platelet counts for massive transfusion was 189 × 10/l (sensitivity 86%, specificity 75%, P < 0.01). We concluded there was no simple relationship between platelet aggregation and injury severity. Our results indicate that high platelet aggregation values are associated with fatality of cerebral injury.