We reviewed the records of 253 patients with head injury who required serial computed tomographic (CT) scans; 123 (48.6%) developed delayed brain injury as evidenced by new or progressive lesions after a CT scan. An abnormality in the prothrombin time, partial thromboplastin time, or platelet count at admission was present in 55% of the patients who showed evidence of delayed injury, and only 9% of those whose subsequent CT scans were unchanged or improved from the time of admission (P < 0.001). Among patients developing delayed injury, mean prothrombin time at admission was significantly longer (14.6 vs. 12.6 s, P < 0.001) and partial thromboplastin time was significantly longer (36.9 vs. 29.2 s, P < 0.001) than patients who did not have delayed injury. If coagulation studies at admission were normal, a patient with head injury had a 31% risk of developing delayed insults. This risk rose to almost 85% if at least one clotting test at admission was abnormal (P < 0.001). We conclude that clotting studies at admission are of value in predicting the occurrence of delayed injury. If coagulopathy is discovered in the patient with head injury early follow-up CT scanning is advocated to discover progressive and new intracranial lesions that are likely to occur.