Major reduction in disability and death from severe subarachnoid hemorrhage (SAH) can probably be brought about more effectively by prevention than by better medical or surgical treatment. Identification of the major risk factors for severe SAH should facilitate the preventive efforts. As risk factors for severe SAH, a patient's age, sex, location and size of aneurysms, hypertension, rebleeding including minor leak, and cigarette smoking were selected. The relation between the incidence of severe SAH, a surgical results, and the risk factors was examined in 81 severe SAH cases. Hypertension, rebleeding from aneurysms, large aneurysm exceeding 1.0 cm/sec in size were closely connected to severe SAH. Treatment of hypertension, acute surgery for aneurysms and aggressive surgery for unruptured large aneurysms were the only hope for achieving substantial reduction in the incidence of severe SAH and for improving the surgical results.