1-3% of patients presenting with sudden onset headache where no blood is visible on a CT scan of the head and with no focal neurological signs will subsequently be shown to have suffered a subarachnoid haemorrhage from a ruptured aneurysm. Because the consequences of misdiagnosing this condition may be dire for the patient, it is important that an appropriate second-line investigation after CT is available. Cerebral angiography is not appropriate on grounds of limited availability, risk and because it is not informative about whether an aneurysm has ruptured or not. Second line investigations have included examination of CSF for red cells, the haemoglobin breakdown products oxyhaemoglobin and bilirubin, and D-dimers and ferritin. Reliance on red cell counts cannot distinguish reliably between an in-vivo bleed and a traumatic puncture and has not been subject to a formal study in CT scan-negative patients. Visual assessment for the presence of oxyhaemoglobin and bilirubin (xanthochromia) is unreliable and must not be used. Spectrophotometry of CSF for oxyhaemoglobin and bilirubin has been subject to large scale formal study, guidelines for specimen collection, analysis and interpretation exist and is the method of choice. Similar formal studies require to be undertaken for chemical determination of bilirubin, quantitation of D-dimers or ferritin to assess their role in the investigative algorithm.