Clinical data available in the first few days after severe head injury have been collected prospectively on 600 patients from Glasgow and the Netherlands and stored on computer; the patients in the two countries were similar in initial severity and in their outcome on a defined scale at 6 months. Calculated predictions of outcome were made in 200 randomly selected cases, using Bayesian statistics to compare the data from each patient with those from the 400 remaining cases whose outcome was known. Confident predictions (greater than 0-97 probability) were made in 44% of cases within the first 24 hours of coma, and in 52-61% when data up to 3 days were available. The higher confidence-rate occurred when prediction was limited to two outcomes (death or survival). Comparison of predicted with actual outcomes showed that 96-98% of confident predictions were correct. More logical clinical decisions should be possible when these predictions are available.