Abstract : I will report on a recent work, joint with Th. Bodineau, L. Saint-Raymond and S. Simonella, in which we develop a rigorous theory of macroscopic fluctuations for a hard sphere gas outside thermal equilibrium, in the Boltzmann-Grad limit : in particular we study deviations from the Boltzmann equation (describing the asymptotic dynamics of the empirical density) and provide, for short kinetic times, both a central limit theorem and large deviation bounds.