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 less than 0.001). Among patients developing delayed injury, mean prothrombin time at admission was significantly longer (14.6 vs. 12.6 s, P less than 0.001) and partial thromboplastin time was significantly longer (36.9 vs. 29.2 s, P less than 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 less than 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.