Study objective: To determine whether tidal expiratory airflow patterns change with increasing airways obstruction in patients with cystic fibrosis. Design: An observational study. Setting: Lung function laboratory. Patients: Sixty-four children and young adults with cystic fibrosis. Measurements: After measuring FEV1 and airways resistance using body plethysmography, each subject was seated and asked to mouth breathe through a pneumotachograph for 2 min. The collected data were analyzed, and three expiratory airflow pattern-sensitive indexes were computed. The first index was derived from the ratio of the time to reach peak expiratory flow to the total expiratory time (tPTEF/tE). The second index, Trs, was an estimate of the time constant of the passive portion of expiration. The third index, S̄, describes the slope of the whole post-peak expiratory flow pattern after scaling. Results: Compared with FEV1, the index tPTEF/tE was a poor indicator of airways obstruction (r2 = 0.15, p = 0.002). Trs showed a strong relationship with the severity of airways obstruction (r2 = 0.46, p < 0.001). Using S̄, the postexpiratory profile could be categorized into three shapes, and provided a good indicator of airways obstruction when linear and concave-shaped profiles occurred (r2 = 0.42, p < 0.001). Convex-shaped flow profiles had to be treated separately and were indicative of normal lung function. Conclusions: In a cross-sectional study of patients with cystic fibrosis, increase in airways resistance above normal is reflected by quantifiable changes in the expiratory airflow pattern.