It is well known that the Serrin condition is a necessary condition for the solvability of the Dirichlet problem for the prescribed mean curvature equation in bounded domains of R n with certain regularity. In this paper we investigate the sharpness of the Serrin condition for the vertical mean curvature equation in the product M n × R. Precisely, given a C 2 bounded domain Ω in M and a function H = H(x, z) continuous in Ω × R and nondecreasing in the variable z, we prove that the strong Serrin condition (n − 1)H ∂Ω (y) ≥ n sup z∈R |H(y, z)| ∀ y ∈ ∂Ω, is a necessary condition for the solvability of the Dirichlet problem in a large class of Riemannian manifolds within which are the Hadamard manifolds and manifolds whose sectional curvatures are bounded above by a positive constant. As a consequence of our results we deduce Jenkins-Serrin and Serrin type sharp solvability criteria. * Supported by CAPES and CNPq of Brazil. † Partially supported by CNPq of Brazil. 2000 AMS Subject Classification: 53C42, 49Q05, 35J25, 35J60.