Serial assessments of glucose tolerance, of glucose and insulin areas during an oral glucose tolerance test (OGTT) and of clinical parameters as well were evaluated retrospectively in seven diabetic cystic fibrosis patients (study group) during the 4-6.3 years that preceded diabetes mellitus diagnosis. The same metabolic and clinical parameters were also evaluated in seven age-matched patients who did not develop diabetes during a similar observation period (control group). In the study group, glucose tolerance was impaired in all patients but one since the first OGTT and glucose areas progressively increased over time, whereas in the control group glucose tolerance remained stable during the whole observation period. A significant and progressive blunting of insulin secretion occurred over time in both groups. Insulin secretion, however, was reduced but not exhausted at diabetes diagnosis. Neither modification of glycosylated haemoglobin levels over time nor serum islet cell antibodies at diabetes onset were observed in the study group. The overall clinical course of the disease was not different in either group and remained stable during the observation period. These results indicate that in cystic fibrosis diabetes mellitus onset is preceded by a long-standing deterioration of glucose tolerance, whilst insulin secretion progressively declines over time, irrespectively of glucose tolerance status. The prediabetic worsening of glucose tolerance is not necessarily linked to a worsening of overall clinical status.