The use of extrinsic positive end expiratory pressure (PEEPe) in patients with auto-PEEP (AP) can reduce the respiratory work during weaning from mechanical ventilation. However, the application of PEEPe can produce a certain level of hyperinflation, an undesirable effect which can limit the efficacy of the reduction of respiratory work. The objective of the present study has been to determine if the increase in end expiratory lung volume (EELV) originated by the PEEPe is related to static lung compliance (SLC). We have studied 14 patients on mechanical ventilation in whom an AP of between 4 and 12 cmH2O was detected. On applying PEEPe equal to half the AP, the EELV increased slightly (77 +/- 64 ml) and was not related to pulmonary compliance. When PEEPe equal to the AP was applied, the EELV increased by 178 +/- 110 ml (range 45-375 ml, p less than 0.05), and there was a significant correlation with SLC (r = 0.659, p less than 0.05). In conclusion, the application of PEEPe equal to the AP causes a moderate increase in EELV. However, in patients with high pulmonary compliance this increase can be more important and must be taken into account when considering the use of PEEPe during weaning.