The invention relates to a hair treatment process comprising the application to the hair of a composition comprising at least one fatty substance using an aerosol device comprising:—a container containing the composition and one or more propellants, it being possible for the propellant(s) to be present in the composition or, in the container, separate from the composition,—a means for dispensing said composition comprising:—a body (3) that is open at its two opposite axial ends,—an engaging part (10) that is open at its two opposite axial ends, at least partially defining a dispensing orifice (12).