The aim of this work is to describe a new instrument based on the negative expiratory pressure technique able to detect expiratory flow limitation (EFL) in patients with respiratory diseases. First, we describe the design details of a virtual instrument able to apply a low pressure at the mouth during tidal expiration, simultaneously measure flow and pressure, and automatically elaborate flow-volume curves. Then, the system accuracy is evaluated investigating normal subjects and patients with chronic obstructive pulmonary disease (COPD). These experimental results revealed higher (p<;;0.001) EFL in COPD patients (58.5+19.9%) than in normal volunteers (1.5+2.5%). These results are in close agreement with the physiology, confirming the high scientific and clinical potential of this system.