Using patient data obtained from the International Cooperative Aneurysm Study that evaluated clinical records from 68 neurosurgical centers in 14 countries, we evaluated the data as to the monthly occurrence of cerebral vasospasm and subarachnoid hemorrhage relative to the latitude of the medical center where the data were gathered. Using the Edwards analysis for cyclic patterns, we examined peak to trough ratios and months of peak to assess the strength and nature of the cyclic variations between December 1980 and July 1983. Of the 3,521 subarachnoid hemorrhage patient records examined, 685 developed some grade of vasospasm as defined symptomatically, angiographically, and by the Fisher Grading Scale. Cyclic analysis demonstrated a strong seasonal occurrence for the incidence of subarachnoid hemorrhage, with a peak in February, in the northern hemisphere. Cerebral vasospasm incidence, after controlling for subarachnoid hemorrhage occurrence, exhibited only a small peak to trough ratio (1.15) relative to the larger ratio (1.74) seen with unadjusted vasospasm data. Age was related to both incidence and location. In populations at high risk for cerebrovascular diseases, climatic conditions may act as synchronizers of pathologic vascular events.