[en] The emission rate of low-mass dileptons at high momenta from a quark-gluon plasma is calculated. A large enhancement compared with the naive Born rate was found. Numerical estimates for relativistic heavy ion collisions at RHIC and LHC suggest, however, that the thermal signal is dominated by the background coming from meson decays and from hadronic bremsstrahlung. (author) 26 refs., 2 figs