The effects of natural convection on the developing forced laminar flow in uniformly heated horizontal tubes are numerically investigated. Detailed results for Rayleigh numbers between 5 x 105 and 3 x 106 were obtained for the temperature and velocity fields showing the effects of the secondary flow on the simultaneous development of the hydrodynamic and thermal boundary layers. Important temperature gradients are observed near the top of the tube, while the transverse velocities reach a maximum intensity and then diminish toward their asymptotic values. As the Rayleigh number increases, the buoyancy-induced secondary flow becomes significant closer to the entrance and the asymptotic Nusselt number increases well above the corresponding value for pure forced flow.