A cosmetic composition for nails containing a concentration less than or equal to 1 wt % of at least one monoaldehyde of the formula (I): R&mdash；CHO, where R is a C5-C12 linear alkyl radical or a C5-C12 alkenyl with an unconjugated double bond to the &mdash；CHO aldehyde group, as nail hardening agent, and provided in the form of a colored or colorless fingernail polish, a polish undercoat, a peelable polish, a beauty oil, an emulsion or an aqueous solution, or a gel. Also the use of aldehydes of the formula (I) for hardening and strengthening nails and to a method for applying same.
