Abstract: A ventilation and air-conditioning system for a high capacity passenger aircraft having plural passenger decks one above another interconnected by stairways or the like, includes two air recycling units (6, 7) respectively incorporating filter units (61, 71) and blower units (62, 72). The inlets of the air recycling units are respectively connected to used air outlets (60, 70) arranged in an upper cabin (2), while the outlets of the recycling units are respectively connected to two air mixing junctions (9 and 10). The first air mixing junction (9) mixes a recirculation airstream from the first recycling unit (6) with an air-conditioned fresh airstream to provide a first mixed airstream into the upper cabin (2) through an air inlet (93). The second air mixing junction (10) mixes a deck transfer recirculation airstream provided by the second recycling unit (7) with an air-conditioned fresh airstream and directs the resulting second mixed airstream into the main cabin (3) through an air supply inlet (103).