To extend Onsager–Machlup's theory, Bertini, De Sole, Gabrielli, Jona-Lasino and Landim proposed a fluctuation theory for the steady states of stochastic nonequilibrium systems, which predicts a temporal asymmetry between a fluctuation and its relaxation. Here, this theory is considered in the context of the nonequilibrium Lorentz gas. This system is deterministic and time reversible, but is chaotic and dissipative, hence its evolution is close to that of irreversible stochastic processes.