Coagulation studies (plasma fibrinogen, ethanol gelation test, and fibrin/fibrinogen degradation product concentration) were done in 150 patients who were admitted after blunt head injury. Results were abnormal in 60 patients and were found to be correlated with the level of consciousness and with the presence of neurological signs. Many of these patients had fractures, but findings in a control group of 26 patients with major fractures without head injury indicate that fractures were not of paramount importance in causing clotting changes. Conclusive evidence of disseminated intravascular coagulation was found in 12 patients. Cases with a fatal clinical course were mostly associated with very high fibrin/fibrinogen degradation product concentrations. Some case histories are reported, confirming the hypothesized correlation between coagulation results and brain tissue destruction rather than brain compression. It was concluded that some degree of disseminated intravascular coagulation in patients with blunt head injury occurs more often than expected and that coagulation studies might have both diagnostic and prognostic value.