The clinical course of intensive care unit (ICU) patients may be complicated by a large spectrum of lower respiratory tract infections (LRTI), defined by specific epidemiological, clinical and microbiological aspects. A European network for ICU-related respiratory infections (ENIRRIs), supported by the European Respiratory Society, has been recently established, with the aim at studying all respiratory tract infective episodes except community-acquired ones. A multicentre, observational study is in progress, enrolling more than 1000 patients fulfilling the clinical, biochemical and radiological findings consistent with a LRTI. This article describes the methodology of this study. A specific interest is the clinical impact of non-ICU-acquired nosocomial pneumonia requiring ICU admission, non-ventilator-associated LRTIs occurring in the ICU, and ventilator-associated tracheobronchitis. The clinical meaning of microbiologically negative infectious episodes and specific details on antibiotic administration modalities, dosages and duration are also highlighted. Recently released guidelines address many unresolved questions which might be answered by such large-scale observational investigations. In light of the paucity of data regarding such topics, new interesting information is expected to be obtained from our network research activities, contributing to optimisation of care for critically ill patients in the ICU.