A theory is introduced to describe the adsorption of rigid rodlike molecules to a planar surface from solution. The configurations of the adsorbed rods are modeled by restricting the possible orientations to three mutually orthogonal directions : two parallel and one normal to the surface. We examine the adsorption isotherm and the most stable configurational state of the adsorbed rods as a function of rod axial ratio, bulk concentration, and adsorption energy. In the case of a strongly attractive wall, the rods are adsorbed parallel to the surface. As the bulk concentration is increased, however, the adsorption isotherm has a kink at a characteristic bulk concentration where the adsorbed rods become preferentially aligned parallel to the surface. For a weaker surface attraction, the adsorbed rods are aligned along a direction normal to the surface. The dependence of the configuration of the adsorbed rods on the temperature is also discussed.