Seasonal variation in the occurrence of cerebrovascular disease has been reported, but data about subarachnoid hemorrhage (SAH) are few and inconclusive. We conducted a nationwide population-based study in Denmark to examine any seasonal pattern of hospitalization and case fatality of SAH. We identified 9,367 patients with SAH and found a modest indication of overall seasonal variation for the risk of hospitalization with this diagnosis. The seasonal pattern, with the highest incidence in January and the nadir in July, was mostly apparent for subjects aged 165 years (peak-to-trough ratio = 1.18; 95% CI 1.04-1.32). There was little difference by sex. The overall 30-day case fatality rate was 38% and showed less seasonal variation. We found evidence of weak seasonal variation in hospitalization for SAH and almost no seasonal variation in a 30-day case fatality rate after SAH. Copyright (C) 2005 S. Karger AG, Basel
