Before the first experiences performed in space, it was already known that the lung and the chest are sensitive to gravity. In the vertical position, the weight of the lung causes top to bottom differences in ventilation, perfusion and gas exchange. Furthermore, the functional residual capacity is position dependent. Thoracic mechanics allows for the explanation of large modifications observed in weightlessness, such as a 40% increase of abdominal respiration. We review few results obtained in weightless conditions and will focus on those where the results were contradicted by predictions. For example, the classical indexes of ventilation inhomogeneity derived from the single and multiple inert gas washout are not sensitive to weightless conditions. These results led to the demonstration of the dependence of these indexes on the structure of the alveolar zone of the lung and found an application on the follow up of lung transplanted subjects and smokers. Contrary to predictions, lung tissue volume decreases after one week in space. The study of aerosol deposition has shown that particles of diameter between 0,5 and one micron diameter penetrate deeper than predicted in the lungs.