Heat transfer and flow visualization experiments were carried out in water to investigate the effect of inclination on natural convection in a parallel-walled channel. Parametric variations were also made of the dimensionless interwall spacing S/H, the mode of heating, and the wall-to-ambient temperature difference (i.e., the Rayleigh number Ras). The heating modes included: (I) both walls heated and maintained at the same uniform temperature, (II) heating only from above, and (III) heating only from below. The Nusselt number results for top-heated channels inclined at an angle θ and for the vertical one-sided heated channel were perfectly correlated by the single parameter (S/H)Rascosθ. On the other hand, the results for bottom-heated channels displayed a separate dependence on S/H, Ras, and θ. The Nusselt numbers for two-sided heating exceeded those for one-sided heating for vertical and moderately inclined channels, with a mixed trend at high inclinations. The global correlation Nus = 0.645[(S/H)Ras]1/4 represented all the results within ± 10 percent. Recirculation zones were present adjacent to the unheated wall of topheated and one-sided heated channels. In the presence of heating from below, secondary flows in the form of longitudinal vortices existed above a threshold Rayleigh number.