In order to define prevalence and incidence of diabetes mellitus in cystic fibrosis, we followed 191 unselected patients above two years of age (median 13.6) in a five-year prospective study with annual oral glucose tolerance tests. The prevalence of diabetes increased from 11 to 24% during the study period with an annual age-dependent incidence rate of 4-9%. Diabetes was diagnosed at a median age of 21 years (range 3-40). At diagnosis of diabetes, hyperglycaemia, fasting hyperglycaemia (> or = 7.8 mmol/l), and increased haemoglobin Alc levels (> 6.4) were present in 33%, 16% and 16% of the diabetic patients, respectively. Impaired glucose tolerance implied a higher risk than normal glucose tolerance for the development of diabetes (odds ratio 5.6). In 58% of cases with impaired glucose tolerance, however, glucose tolerance was normalised at the next annual test. Normal glucose tolerance was found in only 37% of the patients at all five tests. Within this group of patients, median fasting and two-hour post-load plasma glucose concentrations and haemoglobin Alc levels increased by 6-8% during five years. Thus, the prevalence and incidence of diabetes in patients with cystic fibrosis is very high and increases with age. Since symptoms of hyperglycaemia and increased fasting plasma glucose and haemoglobin Alc levels are inconstant findings in newly diagnosed diabetic cystic fibrosis patients, we recommend annual oral glucose tolerance tests in all cystic fibrosis patients above the age of 10 years.