Delayed intracranial hematomas are an important treatable cause of secondary brain injury in patients with head trauma. Early identification and treatment of these mass lesions, which 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 performed in 305 patients. The difference in optical density (ΔOD) at 760 nm between the normal and the hematoma side was measured on admission and then serially during the first 3 to 5 days after injury. On admission, the ΔOD was highly predictive of the initial findings on CT scan. Patients with an epidural, subdural, or intracerebral hematoma had significantly greater ΔOD in the involved brain region than patients with diffuse brain injury. For the extracerebral hematomas, the ΔOD was significantly related to the size of the hematoma (r2=0.55 and 0.77 for subdural and epidural hematomas, respectively). Fifty-nine (19%) of the patients developed some type of late hematoma: there was an intracerebral hematoma in 29 patients, an extracerebral hematoma in 7 patients, and a postoperative hematoma in 23 patients. Thirty-three of the late hematomas were large enough to require surgical evacuation. The hematomas appeared between 2 and 72 h after admission. In 55 of the 59 patients, an increase in the ΔOD to >0.10 occurred prior to an increase in intracranial pressure or a change in the neurological examination. Early diagnosis using NIRS may allow early treatment and reduce secondary injury caused by delayed hematomas.