Thirty‐two patients with cystic fibrosis and repeatedly normal fasting blood glucose underwent oral glucose tests and islet‐cell antibody assessments on two occasions approximately two years apart. Fourteen patients underwent two iv glucose tolerance tests also. Although in the group as a whole mean glucose areas in response to the oral test remained substantially unmodified over the two‐year period, the prevalence of glucose tolerance abnormalities increased from 37.5 to 50%. Insulin output in response to both oral and iv tolerance tests decreased over time. Worsening of insulin secretion and/or of glucose tolerance was never accompanied by deteriorating clinical status. Islet‐cell antibodies were detected in no patients, even in those who developed a diabetic glucose tolerance. These results support, on a longitudinal basis, the view of a progressive impairment of B‐cell function in cystic fibrosis, which may precede the onset of metabolic abnormalities and is not triggered by aut