Predictors of one-year outcome were studied in patients treated for ruptured intracranial aneurysm. A total of 929 patients, who were treated conservatively or surgically, and 839 patients as a part of this population who were surgically treated, were randomly divided into two groups in order to create predictive models by logistic regression and to validate them. The models were derived from two-thirds of these two patient groups and the remaining one-thirds were used for validation. The pre-operative variables of both conservatively and surgically treated patients were studied by Model A. The pre-operative Grade (Hunt and Hess), age, and the presence of vasospasm on angiography were the three most important predictors of the one-year outcome (Glasgow Outcome Scale 1-2/3-5). Model B consisted of pre- and per-operative, and Model C pre-, per-, and post-operative variables collected from the surgically treated group. The pre-operative Grade, ligation of a major vessel and age were the three most powerful determinants of outcome in Model B. In Model C high Grade, post-operative CT-hypodensities and ligation of a major vessel were most closely associated with poor outcome. Model A, based on pre-operative data, most accurately predicted good outcome. All the 59 patients in the validation sample (n = 310) who were predicted to have a less than 5% probability of poor outcome had a favourable outcome (positive and negative predictive value 100%). Model C determined that 22 of 280 patients would have a more than 80% probability of poor outcome.(ABSTRACT TRUNCATED AT 250 WORDS)