The lung collapse pattern was studied in 10 intubated adult patients with acute respiratory distress syndrome (ARDS). Tracheal pressure (Ptr), airway pressure (Paw), airflow (V’) and volume (V) at four different PEEP levels (0, 5, 10, 15 cm H2O) were continually recorded. Expiratory volume (passive expiration) was divided in four equal contiguous segments; time constant (τe) and static respiratory compliance (Crs) of each segment were separately calculated using linear regression analysis of V/V’ and V/Ptr ratios, respectively. The τe/Crs ratio was used for the calculation of total expiratory resistance (Rtot, the sum of the respiratory system resistance [Rrs], plus the endotracheal tube airflow resistance [Rtube], plus the ventilator circuit resistance [Rvent]). In the presence of expiratory flow limitation, Rrs was assumed equal to RTOT. If no expiratory flow limitation was documented, Rrs was calculated as Rtot minus the sum of Rtube plus Rvent. The time constant of the respiratory system (τers) is represented by the product of Rrs and Crs. At zero PEEP (ZEEP), τers was shown to increase significantly from the beginning toward the end of expiration (Δτers 55.5±31%) due to the marked increase in Rrs (ΔRrs 63.9±45%). When PEEP was applied, Rrs was maintained relatively constant throughout expiration, the collapse rate of the respiratory system increased and the difference in τers from the beginning toward the end of expiration reduced. Conclusively, the lungs of ARDS patients demonstrate a nonhomogeneous collapse rate during expiration as a result of a marked increase in respiratory system resistance observed in low lung volumes. The use of PEEP leads to a homogeneous and higher lung collapse rate. Pneumon 2002, 15(2)147-157.