The present invention relates to the use, for the dyeing with lightening effect of human keratin substances, and in particular of human keratin fibres such as the hair, of a composition comprising at least one fluorescent cyanine dye. The present invention allows human keratin substances to be both dyed and at the same time lightened without detriment to said substances. The fluorescent dyes employed in the context of the invention also have high tinctorial affinity for keratin substances and good properties of resistance towards external agents, and especially towards shampoos.