The prevalences of impaired glucose tolerance (IGT), diabetes mellitus and late diabetic complications were studied in all Danish cystic fibrosis (CF) patients. A total of 311 CF patients were identified with an estimated ascertainment rate above 98%. Glucose tolerance was classified in 278 (89%) patients: the prevalences of IGT and diabetes mellitus were 13.7% (38 patients) and 14.7% (41 patients), respectively, with no sex differences. The prevalence of diabetes mellitus increased with age but not with the severity of CF as compared with age- and sex-matched non-diabetic CF patients. Diabetes was diagnosed at a median age of 20 years (range 3-40 years) and the duration of diabetes was 1.7 years (0.1-17 years). Twenty-eight of the diabetic patients (70%) were treated with insulin, on average 20 (4-90) IU per day. Late diabetic complications were identified in 4 patients (10%) with a duration of diabetes mellitus of 1-17 years: background retinopathy (2 patients), diabetic nephropathy (1 patient), microalbuminuria (1 patient) and neuropathy (2 patients). Thus diabetic CF patients are probably not less prone to develop late diabetic complications than patients with other types of diabetes of equally long duration and comparable glycemic control.