High-T-c superconductors have Fermi energies E-F much smaller than conventi
onal metals comparable to phonon frequencies. In such a situation nonadiaba
tic effects are important. A. generalization of Eliashberg theory in the no
nadiabatic regime has previously been shown to reproduce some anomalous fea
tures of the high-T-c superconductors as for istance the enhancement of T-c
or the isotopic effects on T-c and m*. In this contribution we address the
issue of the symmetry of the gap in the context of nonadiabatic supercondu
ctivity. We show that vertex corrections have a momentum structure which fa
vours d-wave superconductivity when forward scattering is predominant. An a
dditional increase of T-c is also found.