BACKGROUND: Patients with moderate and severe traumatic brain injury (TBI) are at risk for secondary brain insults such as thrombocytopenia and coagulopathy. This study assessed the development of thrombocytopenia and coagulopathy at admission and within the subsequent 72 hours after TBI. METHODS: Blunt trauma patients with moderate or severe TBI and an extracranial Abbreviated Injury Scale score less than 3 were reviewed. Data collection included initial and subsequent prothrombin time, partial thromboplastin time, and platelet values. RESULTS: On initial evaluation, thrombocytopenia was present in 14% and coagulopathy in 21% of patients. By the third day, thrombocytopenia and coagulopathy increased to 46% and 41%, respectively. Of patients who died, 67% had thrombocytopenia and 62% had coagulopathy. CONCLUSION: Patients with moderate and severe TBI are at risk for thrombocytopenia and coagulopathy, not only at admission but also on subsequent laboratory examination. Repeat laboratory evaluation is warranted even if initial results are normal in this population.