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A long pre-diabetic phase of abnormal glucose tolerance is described in subjects with cystic fibrosis (CF) since childhood. Objective Aims of the study were to compare oral glucose tolerance test (OGTT) and continuous glucose monitoring system (CGMS) in the diagnosis of altered glucose metabolism, and to evaluate longitudinally the role of CGMS in predicting glucose metabolism deterioration in children with CF. Methods Seventeen children with CF and 14 controls were enrolled (mean age 13.3±3.0 years). All subjects underwent OGTT and CGMS registration. On the basis of OGTT children were classified as normal glucose tolerance, impaired glucose tolerance (IGT), IGT plus at least one glucose value above 200 mg/dl at intermediate OGTT points (IGT+200), CF related diabetes (CFRD). HbA1c, glucose area under the curve, insulin sensitivity, insulinogenic and disposition index were also considered. Subjects with CF repeated OGTT after 2.5 years. Results Baseline OGTT revealed 3/17 (7.6%) children with CF with at least one glucose value above 200 mg/dl (1 CFRD, 2 IGT+200), while CGMS 6/17 (35.3%) with glucose excursions abov