The presence of various heme derivatives can be demonstrated spectrophotometrically in colorless cerebrospinal fluid (CSF). Because of the high sensitivity of the method, it may detect compounds that reflect a "traumatic tap" rather than a disease process. However, the presence of methemoglobin excludes the possibility of a hemorrhagic CSF being caused by traumatic lumbar puncture. Here we describe a highly sensitive spectrophotometric method involving measurement at the Soret band (400-420 nm) to detect methemoglobin (greater than or equal to 15%) in trace amounts of hemoglobin mixture (less than 0.3 mumol/L). We demonstrated methemoglobin in colorless CSF samples in 9% of 454 patients with cerebrovascular pathology and in 4% of 449 patients with other neurological diseases (n = 449). In a group of 21 patients with verified acute cerebral hematomas, methemoglobin was confirmed in 66% of colorless CSF samples after ultrafiltration. We conclude that routine spectrophotometric analysis of all CSF samples is very useful, allowing detection of xanthochromic compounds in patients with small cerebral and subdural hematomas as well as in those with minimal subarachnoid hemorrhages, hemorrhagic infarctions, or bleedings from aneurysms and neoplasms.