To define more precisely the factors determining the allocation of resources to critically ill patients, we asked physicians to estimate at the time of admission the short-term prognosis of patients who accounted for 1831 admissions to a medical intensive-care and coronary-care unit. We then examined the relations between this prognosis, the actual outcome, and the resource expenditure during a single hospitalization. We found that the care of nonsurvivors involved a significantly higher mean expenditure than did the care of survivors (P less than 0.01). Among nonsurvivors, expenditure positively correlated with the probability of survival estimated at the time of admission (P less than 0.001). Among survivors, expenditure negatively correlated with the probability of survival (P less than 0.001). Among both nonsurvivors and survivors, total expenditure and expenditure per day were greatest for patients whose outcome were most unexpected. We conclude that prognostic uncertainty is important in determining resource expenditures for the critically ill. This factor warrants greater consideration in future studies of expenditure for the care of catastrophically ill patients.