A finite element code is used for detailed analysis of mixed-convection flow in a horizontal channel heated from the side walls. The Reynolds number typically is Re=500, and the Grashof number varies around Gr=105. Firstly, the numerical code is validated by a quantitative comparison with results of particle-tracking experiments for the velocity and liquid crystal measurements for the temperature. It is concluded that the agreement between the numerical and experimental data is satisfactory and that the finite element approximation employed can be used in analyzing mixed-convection flow problems in complex geometries. Secondly, the velocity and temperature fields are further described by means of isovelocity and isotemperature lines. The results are quantified by the relative kinetic energy of secondary flow, the Fanning friction factor, and the Nusselt number as a function of the streamwise position. It seems that the resulting secondary flow induced by buoyancy forces causes a substantial increase in heat transfer, knowledge of which is of importance for the design of compact heat exchangers.