  We calculate the exact values of the holomorphic observables of {\cal N}=4
supersymmetric SU(N) Yang-Mills theory deformed by mass terms which preserve
{\cal N}=1 SUSY. These include the chiral condensates in each massive vacuum of
the theory as well as the central charge which determines the tension of BPS
saturated domain walls interpolating between these vacua. Several unexpected
features emerge in the large-N limit, including anomalous modular properties
under an SL(2,Z) duality group which acts on a complexification of the 't Hooft
coupling \lambda=g^{2}N/4\pi. We discuss our results in the context of the
AdS/CFT correspondence.
