Intestinal permeability to 51Cr-EDTA was studied in 20 children with cystic fibrosis (CF) and a mean age of 10.7 years, in 7 control adults and in 11 control children. 51Cr-EDTA urinary excretion (mean +/- SD) expressed as a percentage of the orally administered activity was: control children: 1.59 +/- 0.55%, control adults: 1.47 +/- 0.62%, CF patients: 10.7 +/- 8.6%. The difference between CF patients and control children on one hand, between CF patients and control adults on the other hand, was statistically significant (p less than 0.001). Only 3 CF patients had values within the limits of those observed in controls. A statistically significant correlation (p less than 0.01) was observed between the 51Cr-EDTA urinary excretion and steatorrhea. No correlation was found between 51Cr-EDTA urinary excretion and the following parameters: age, sex, weight, height, Shwachman score, liver cirrhosis, oral administration of a mucolytic agent. The eventual, especially nutritional, consequences of this increase of the intestinal permeability in the course of CF remain to be clarified.