The aim of this study was to evaluate the prevalence of impaired glucose tolerance or diabetes mellitus in 99 patients (53 M, 46 F; mean age 10.5 +/- 6.9 years), with cystic fibrosis. Glucose tolerance was evaluated in all patients without overt diabetes using the oral glucose tolerance test (OGTT). Six patients showed a pathological OGTT and 2 patients had insulin-requiring diabetes mellitus. The mean age of the patients with impaired glucose tolerance was significantly higher than that of the subjects with normal glucose metabolism (p less than 0.0001). Patients with overt diabetes mellitus were the oldest subjects in the study group.