The treatment of any abnormal state of health depends on an accurate assessment of the natural history of the underlying disease process. The term natural history is used frequently in medical writings, but it is doubtful that the complete extent of a given disease can be covered in any single survey. With respect to nontraumatic subarachnoid hemorrhage, one cannot hope to cover the entire spectrum of this disease through reporting the results obtained in referral centers because of the screening and selection of patients who come to such centers. Thus, the original Cooperative Study of Intracranial Aneurysms and Subarachnoid Hemorrhage necessarily dealt with a select group of patients who had survived the original insults long enough for subarachnoid hemorrhage to be diagnosed and to arrive at a tertiary care center. The natural history was further confounded by the selection of certain patients (usually the prime neurological candidates) for surgical treatment.