The invention relates to a heat exchanger for motor vehicles, having a large number of flat tubes through which a liquid cooling medium can flow, and having corrugated fins which are associated with these flat tubes and to which environmental air or other media can be applied. The flat tubes having indentations pointing inward on at least one of their flat faces. Heat transfer between the core flow of the cooling medium and the flat tube walls is improved, and the power density of the heat exchanger is thus increased. The indentations are preferably in the form of elongated vortex generators.