The impact of warning leaks on management results in patients with aneurysmal subarachnoid hemorrhage (SAH) was evaluated in this prospective study. In a consecutive series of 422 patients with aneurysmal SAH, 84 patients (19.9%) had an episode suggesting a warning leak; 34 (40.5%) of these patients were seen by a physician without the condition being recognized. The warning leak occurred less than 2 weeks before a major SAH in 75% of the patients. A good outcome was experienced by 53.6% of patients who had a warning leak versus 63.3% of those who had no warning leak. In a subgroup of patients who had an interval of 3 days or less from warning leak to SAH, only 36.4% had a good outcome. The proportion of patients in good neurological condition (Hunt and Hess Grades I and II) who had a good outcome was 88.1% in the group with no warning leak versus 53.6% in the group whose SAH was preceded by a warning leak. A difference of 35% between these two groups reflects the impact of an undiagnosed warning leak on patient outcome, based on the assumption that patients with a warning leak had clinical conditions no worse than Hunt and Hess Grade II at the time of the episode. In the subgroup of patients with the short interval between warning leak and SAH, the difference was almost 52%. The difference in outcome also reflects the potential improvement in outcome that can be achieved by a correct diagnosis of the warning leak. If the correct diagnosis is made in patients seeking medical attention due to a warning leak, favorable outcomes in the overall management of aneurysmal SAH are estimated to increase by 2.8%. An active diagnostic attitude toward patients experiencing a sudden and severe headache is warranted as it offers a means of improving overall outcome in patients with SAH.