Cerebrospinal fluid (CSF) spectrophotometry for bilirubin is a highly sensitive test in the diagnostic work up of a suspected subarachnoid hemorrhage (SAH).We report two cases suffering from an aneurysmal SAH in which extraventricular drainage for acute hydrocephalus was required. Longitudinal analyses of the CSF samples demonstrated that CSF bilirubin was detectable in all cases during the first week, becoming undetectable in one case in the second week. Importantly, CSF ferritin levels rose substantially (> 1,000 ng/ml) after 6 days, peaking around 3,000 ng/ml after 2 weeks (normal upper reference range 12 ng/ml). In both cases blood was visible on the initial CT brain scan, disappearing on a later scan.CSF ferritin levels may be an important additional laboratory test in the diagnostic work-up of patients with a suspected SAH. CSF ferritin levels may prove particularly helpful in cases with late presentation if the CT brain scan is normal and CSF bilirubin level is undetectable.