ABSTRACT Objective: The aim of this study was to determine what changes occur in previously described tidal expiratory flow patterns when extrathoracic resistance is added. Methodology: A total of 16 subjects with normal lung function and 15 patients with chronic COPD, were studied. The following measurements were made before and after the addition of an extrathoracic expiratory resistance (EER; 0.89 kPa/L s at a flow of 0.5 L/s) during uncoached tidal breathing: expiratory time (tE), frequency (f), expiratory time to maximum flow/total expiratory time (tPTEF/tE), tidal maximum expiratory flow (PEFTIDAL), and time constant of the respiratory system calculated from the passive portion of tidal expiration (trsTIDAL). Post peak profiles were categorized as convex, linear or concave and time for flow to fall to 80% and 20% of maximum (t80 and t20) were measured. Results: Tidal indices tE, f and PEFTIDAL, were similar in the two groups, but tPTEF/tE was greater and trsTIDAL was less in the normal subjects than in the COPD patients. Addition of EER in both groups caused PEFTIDAL to decrease and trsTIDAL, t80 and the percentage of convex profiles to increase. tPTEF/tE increased in the COPD patients but was unchanged in the control group. Conclusion: After addition of EER during tidal breathing, expiratory flow in COPD patients resembled that seen in normal subjects.