We have determined the magnetic field dependence of the electron density of states at the Fermi level, N-F(H), by specific heat measurements on single-crystal YBCO. The total specific heat includes contributions from phonons and impurities, as well as the electronic specific heat. The electronic specific heat is best described by a model which includes two characteristics of d-wave superconductors: a T-2 term in zero held, and a linear term that corresponds to N-F(H) proportional to (H/H(c)2)(1/2), predicted by Volovik for superconductors with lines of nodes in the gap.