In the Danish Aneurysm Study 1076 patients (pts.) were admitted with an aneurysmal subarachnoid hemorrhage in the 5-year period 1978-83. A warning leak (WL), defined as a sudden episode of headache, vomiting, nuchal pain, dizziness or drowsiness, was identified in 166 pts. (15.4%). In 99 of these the episode was evaluated by a physician but misdiagnosed. A 2-year follow-up examination of the 99 pts. showed that 30 pts. had a normal mental outcome and 43 pts. were dead. If these patients were correctly diagnosed after the WL, when they were in Hunt grade 1-2, the outcome-figures would probably have been significantly better. A theoretical transfer of the outcome-probabilities for pts. in Hunt grade 1-2 to the above mentioned 99 pts. would result in 66 pts. with a normal mental outcome and 25 dead pts. This shows the importance of recognition of a WL episode.