The management of aneurysmal subarachnoid haemorrhage has recently changed considerably. Emergency admission to specialized centres and early surgery have become common practice. In addition, the use of nimodipine has gained widespread acceptance. Little data are available concerning the frequency and temporal profile of reruptures under the current policies. The case histories of 387 patients treated for aneurysmal subarachnoid haemorrhage between January 1984 and March 1992 were reviewed with regard to the incidence of in-hospital reruptures. All patients were managed according to the same protocol including a policy of individually timed early surgery and intravenous nimodipine. A total of 44 first in-hospital rebleeds were observed during the waiting period. Two percent of the patients admitted on the day of haemorrhage had a rebleed on the same day after admission to the hospital. No rebleeds were observed on the day after subarachnoid haemorrhage. Rebleed rates on day 2 and 3 were also low with 0.6 and 0.8% of the population with an unclipped aneurysm. For the following 10 days, the daily rate of rerupture increased. A further peak was observed during the 4th week. Using life-table methods, the cumulative rate of rebleeds was calculated as 23% within 2 weeks and 42% within 4 weeks. Although patients suffering rebleeds differed in several respects from patients without rebleeds, most of the differences could be identified to be a consequence of a selection bias resulting in a longer period of exposure to the risk of rerupture for certain subgroups. Only patients suffering a loss of consciousness after the initial subarachnoid haemorrhage were definitively exposed to a higher daily risk of rerupture.(ABSTRACT TRUNCATED AT 250 WORDS)