Methane production was studied in 28 cystic fibrosis patients aged 3-16 years and in 290 healthy children. The percentages of methane producers in the cystic fibrosis and control groups were 60.7 and 20.6%, respectively. In the patient group there was no difference between methane producers and nonproducers with respect to the degree of malabsorption or the administration of pancreatic supplements or antibiotics. We hypothesize that the high proportion of methane producers in cystic fibrosis patients may be associated with the presence of specific substrate(s) (glycoprotein?) in their intestinal contents. Changes of the intestinal microenvironment may favor the development of a methanogenic flora.