We determine the effective prepotential for N = 2 supersymmetric SU(Nc) gauge theories with an arbitrary number of flavors Nf < 2Nc, from the exact solution constructed out of spectral curves. The prepotential is the same for the several models of spectral curves proposed in the literature. It has to all orders the logarithmic singularities of the one-loop perturbative corrections, thus confirming the non-renormalization theorems from supersymmetry. In particular, the renormalized order parameters and their duals have all the correct monodromy transformations prescribed at weak coupling. We evaluate explicitly the contributions of 1- and 2-instanton processes.