Background Normal values (>80%) of Forced Expiratory Volume in one second (FEV1) in patients with Cystic fibrosis (CF) may lead to the interpretation that there is no lung disease. This study is a comprehensive analysis of lung involvement in CF patients having normal FEV1. Methods Patients were recruited from two CF Centers: Hadassah Medical Center, Jerusalem and Vall d’ Hebron Hospital, Barcelona. Lung disease was assessed by lung clearance index (LCI), chest CT-Brody Score, respiratory cultures, number of pulmonary exacerbations (PEx) and days of antibiotic treatment in the year prior to the assessment. Results Of the 247 patients, 89 (36%) had FEV1 ≥80% and were included in the study (mean age 17.6 range 4.25-49 years). Chronic P. aeruginosa infection was found in 21%, and 31% had at least one major PEx in the year prior to the study. Abnormally elevated LCI was found in 86% of patients, ranging between 7.52-18.97, and total Brody score (TBS) was abnormal in 92% (range 5.0-96.5). Patients with chronic P. aeruginosa had significantly higher LCI (p=0.01) and TBS (p=0.01) which were associated with more major PEx (p = 0.04 and p<0.001, respectively) and more days of intravenous (IV) antibiotic treatment in the preceding year (p=0.03 and p=0.002, respectively). Conclusions Most CF patients with normal FEV1 have already physiological and structural lung abnormalities which were associated with more PEx and IV antibiotic treatment.