The synthetic peptide N-benzoyl-L-tyrosyl-p-aminobenzoic acid is specifically cleaved by chymotrypsin to Bz-Ty and PABA. The liberated PABA is absorbed and excreted in the urine. Accordingly, PABA recovery reflects intraluminal chymotrypsin activity and is an index of exocrine pancreatic function. This test was evaluated in 24 patients with cystic fibrosis to determine its role in the diagnosis of exocrine pancreatic insufficiency. Cumulative percent PABA recovery in six hours was significantly lower in CF patients compared with the control group. No overlap was noted between the two groups. There was good correlation between PABA recovery, fecal chymotrypsin activity, and coefficient of fat absorption. These findings indicate that PABA recovery is significantly reduced in patients with CF and steatorrhea and may prove a practical and reliable test of pancreatic insufficiency.