INTRODUCTION: In clinical practice, the occurrence of aneurysmal subarachnoid haemorrhage (SAH) often coincides with a particular season. Our objective was to examine seasonal variations in hospital admissions due to aneurysmal SAH. METHODS: The study population consisted of 489 patients with aneurysmal SAH who were admitted to the Department of Neurosurgery, University Hospital of Zurich, Switzerland, between 1st of January 1996 and 31st of December 2002. Statistical significance of seasonal variation was determined by applying Roger's r test. RESULTS: Statistically significant seasonal variation was only found among patients younger than 60 years, showing a first peak in spring and second lower peak in autumn (Roger's r=6.89, p<0.05). A borderline significance was found in men younger than 60 years (Roger's r=5.96, p=0.051). A trend was observed in patients presenting with Fisher grade 1-2 (Roger's r=5.70, p=0.058). CONCLUSIONS: Previous studies from different countries have shown significant seasonal variations, with the peak period for aneurysmal SAH differing widely. There appears to be some link between aneurysmal SAH and the season of the year or variations in weather conditions. Further investigations are desirable to evaluate which weather or climatic parameters correlate well with SAH.