During forced vital capacity maneuvers in subjects with expiratory flow limitation, lung volume decreases during expiration both by air flowing out of the lung (i.e.: exhaled volume) and by compression of gas within the thorax. As a result, a flow-volume loop generated by using exhaled volume is not representative of actual flow volume relationship. We present a novel method to take into account the effects of gas compression on flow and volume in the first second of a forced expiratory maneuver (FEV₁). In addition to oral and esophageal pressures, we measured flow and volume simultaneously using a volume-displacement plethysmograph and a pneumotachograph in normal subjects and patients with expiratory flow limitation. Expiratory flow versus plethysmograph volume signals was used to generate a flow-volume loop. Specialized software was developed to estimate FEV1 corrected for gas compression (NFEV1). We measured reproducibility of NFEV₁ in repeated maneuvers within the same session and over 6-month interval in patients with chronic obstructive pulmonary disease (COPD). Our results demonstrate that NFEV₁ significantly correlated with FEV₁, peak expiratory flow, lung expiratory resistance, and total lung capacity. During intra-session, maneuvers with highest and lowest FEV1 showed significant statistical difference between mean FEV1 (p <0.005) while NFEV1 from same maneuvers were not significantly different from each other (p >0.05). Furthermore, variability of NFEV₁ measurements over 6-month was less than 5%. We concluded that our method reliably measures the effect of gas compression on expiratory flow.