Statistical discriminant analysis is applied to 41 concomitant variables obtained during the first year of study of 226 patients with cystic fibrosis. A discriminant function based on six variables is developed which can be used as a predictive index. This index estimates the probability of 5-year survival for an individual patient and can also be used to classify patients into one of two groups: (1) will live for 5 years or (2) will die within 5 years. Speculation A classification rule for cystic fibrosis is presented which correctly assigns 90% of the 5-year survivors and 84% of those who expire. Such a classification scheme is useful for clinical and research purposes.