The number of adults with cystic fibrosis (CF) receiving care from the US Cystic Fibrosis Centers has been increasing at an average rate of about 200 patients each year. A model based on the continuation of this rate of increase predicts that almost 4,000 adults will be receiving care at the centers by the end of 1985. Four other models developed are based on continuation of a steady state with two variations: a 50% reduction in mortality and universal newborn screening. These models show the boundaries of an equilibrium population to be between 24,000 and 64,000 patients, with 7,000 to 18,000 adults.