We investigated 91 patients with non-traumatic subarachnoid hemorrhage (SAH) in whom no aneurysm was found on initial angiography. In 31 of these patients CT did not show subarachnoid blood. A typical feature was a localized blood clot in the prepontine or interpeduncular cistern. Follow-up was obtained 6 months to 11.8 years after SAH by personal re-examination (n = 49) or questionnaire sent to patients or their doctors. 79 patients were free of neurological signs, 3 had a mild and one a severe neurological deficit. Five patients had died from SAH. Those patients who died or had complications often had blood in the frontal basal interhemispheric fissure. None of the 3 therapeutic regimens applied proved to be superior. We conclude that prognosis is good, but an unidentified aneurysm must be considered and repeat angiogram is warranted if blood is found in the anterior part of the basal cisterns. When the hemorrhage is in the prepontine cistern, repeat angiogram is not necessary.