Clinical studies have documented the importance of secondary brain insults in determining neurologic outcome after head injury. Delayed intracranial hematomas are one of the most easily remediable causes of secondary injury if identified early, but can cause significant disability or death if not promptly recognized and treated. Early identification and treatment of these lesions that appear or enlarge after the initial CT scan may improve neurological outcome. Serial examinations using near-infrared spectroscopy (NIRS) to detect the development of delayed hematomas were obtained in 167 patients. The difference in absorbance of light (deltaOD) at 760 nm between the normal and the hematoma side was measured serially during the first 3 days after injury. Twenty-seven (16%) of the patients developed some type of late hematoma: an intracerebral hematoma in 8 patients, an extracerebral hematoma in 6 patients, and a postoperative hematoma in 13 patients. Eighteen of the delayed hematomas caused significant mass effect and required surgical evacuation. The hematomas appeared between 2 and 72 h after admission. In 24 of the 27 patients, a significant increase (>0.3) in the deltaOD occurred prior to an increase in intracranial pressure or a change in the neurological examination, or a change on CT scan. Early diagnosis using MRS may allow early treatment and reduce secondary injury caused by delayed hematomas.