The Glasgow coma score (GCS) during days 1-6 after cardiac arrest was used to predict neurological outcome in 360 resuscitated victims of out-of-hospital cardiac arrest. A predictive rule based on the best GCS of 216 patients resuscitated in 1983-84 (prediction group) was constructed, and its predictive power was tested on 133 patients treated in 1985 (test group). Neurological outcome was correctly predicted 2 days after cardiac arrest in 80% of the prediction group, with a best GCS of 10 or above and 4 or below as cutoff points. For patients with a best GCS of 5-9, prediction of outcome was possible 6 days after cardiac arrest, with a best GCS of 8 during the first 6 days as the single cutoff point. The rule was then validated in the test group: the sensitivity was 96%; the specificity 86%; the negative predictive value 97%; and the positive predictive value 77%. These data suggest that this simple GCS-based rule can be helpful in predicting outcome in patients resuscitated after out-of-hospital cardiac arrest, but confirmation of these data is required in a prospective study in a larger number of patients.