Objective Patients without a subarachnoid haemorrhage (SAH) on brain CT scan (CT-negative), but a lumbar puncture (LP)-proven SAH, are a challenging patient category. The optimal diagnostic approach is still a matter of debate. Also, there is little knowledge on the probability of finding an underlying vascular lesion.

Design In this observational study, a consecutive cohort of 94 patients with CT-negative, LP-positive SAH was prospectively collected between 1998 and 2013. The yield of diagnostic modalities as well as patient outcome was studied. In addition, risk factors for the presence of a vascular lesion were analysed.

Results In 40 patients (43%), an intracranial vascular abnormality was detected: 37 aneurysms and three arterial dissections. Female gender was significantly associated with detection of a vascular lesion. Time between ictus and diagnosis of SAH was not associated with the presence of vascular pathology. Overall, 99% of patients had a modified Rankin Score of 0–2 after a median follow-up of 72 months. The yield of additional digital subtraction angiography in patients with a negative CT angiography was zero.

Conclusions In this study, the chance of finding a vascular lesion in a patient with CT-negative, LP-positive SAH was 43%, underlining the need for an adequate diagnostic workup. In general, the patient outcome was favourable. Female gender was found to be predictive for detecting a vascular lesion. In contrast with previous reports, the interval between ictus and LP was not associated with the presence of an aneurysm.