Urinary excretion of lactulose, mannitol, and 3-0-methylglucose, following oral administration (5 g, 5 g, and 2 g, respectively, in 100 ml H2O; 80 ml/m2), has been measured in subjects with cystic fibrosis (CF) (22), Shwachman syndrome (3), chronic pancreatitis (3), and normal controls (46). Mean lactulose excretion was increased 10-fold in CF (p less than 0.001), and two-fold in other disorders associated with pancreatic insufficiency (PI) (p less than 0.05). Mean mannitol excretion was 1.6 times greater in CF (p less than 0.001), compared with controls, but was reduced in other forms of PI (p less than 0.03). The mean lactulose/mannitol excretion ratio was increased in all types of PI (p less than 0.001). There were no significant differences in 3-0-methylglucose excretion. This study confirms the large increase in lactulose absorption recently reported in CF and also demonstrates increased absorption of mannitol; these changes are different than those in other forms of PI. This study provides further evidence for a specific abnormality of the mucosal barrier to the absorption of passively absorbed, water soluble molecules in CF.