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An important question is if the gap in the high temperature cuprates has Typeset using REVT E X 1

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Monte Carlo results for a two-dimensional Hubbard model in the intermediate-coupling regime U =4t are compared with a diagrammatic spin-fluctuation approximation.Simulations on an 8X8 lattice doped away from half filling were carried out down to temperatures of order -'o of the bandwidth. Results for the spin susceptibility y(q, ice ), the electron self-energy X(p, ice"), various pair-field susceptibilities, and the irreducible particle-particle scattering vertex l (p,ico"~p,ito") were obtained. A random-phase approximation for y(q, ice ) with a renormalized Coulomb coupling U is shown to provide a fit to the Monte Carlo data. A similar approximation for the Berk-Schrieffer spin-fluctuation interaction also provides a reasonable fit to the self-energy X(p, im") in the region explored by the Monte Carlo data. However, a similar approximation for the irreducible particle-particle interaction failed to reproduce the Monte Carlo results. Higher-order vertex corrections were calculated, but significant discrepancies with Monte Carlo results for I remain.