The three-dimensional laminar mixed convection boundary layer on a vertical heated surface exposed to a horizontal cross flow was investigated by using a full three-dimensional finite-difference code, local similarity, and a first-order perturbation solution. The full three-dimensional equations were solved by a modified version of the box scheme of Keller and Cebeci. The results indicate that natural convection dominates near the lower leading edge of the surface and at large values of the horizontal coordinate, whereas forced convection dominates near the vertical leading edge and at large values of the vertical coordinate. In comparing the approximate methods of local similarity and first-order perturbation, where the perturbation parameter was taken to be α = gcβ ΔTK/w2∞,, it was found that local similarity yielded more accurate results and required less computer time and programming effort.