Abstract. We consider the energy-critical semilinear focusing wave equation in dimension N = 3, 4, 5. An explicit solution W of this equation is known. By the work of C. Kenig and F. Merle, any solution of initial condition (u0, u1) such that E(u0, u1) < E(W, 0) and ∇u0 L 2 < ∇W L 2 is defined globally and has finite L 2(N +1) N −2 t,x -norm, which implies that it scatters. In this note, we show that the supremum of the L 2(N +1) N −2 t,x -norm taken on all scattering solutions at a certain level of energy below E(W, 0) blows-up logarithmically as this level approaches the critical value E(W, 0). We also give a similar result in the case of the radial energy-critical focusing semilinear Schrödinger equation. The proofs rely on the compactness argument of C. Kenig and F. Merle, on a classification result, due to the authors, at the energy level E(W, 0), and on the analysis of the linearized equation around W .