Numerical results are presented for the effects of buoyancy on the hydrodynamic and thermal parameters in the laminar, vertically upward flow of a gas in a parallel-plate channel. Solutions of the governing parabolic equations are obtained by the use of an implicit finite difference technique coupled with a marching procedure. It is found that buoyancy dramatically increases the hydrodynamic entry length but diminishes the thermal development distance. At a fixed wall temperature difference ratio, buoyancy enhances the heat transfer on the hot wall but has little impact on the cool wall heat transfer. Flow reversal is observed in some cases. Based on an analytical solution for fully developed flow, criteria for the occurrence of flow reversal are presented. The present numerical solutions yield results that asymptotically approach those from the analytical solution.