Using diffusion theory, we give the first results for a three-dimensional model of the proton radiation belt. This is based on the numerical solution of a diffusion equation which takes into account the cosmic ray albedo neutron decay source, deceleration of protons by the free and bounded electrons of the medium, the charge exchange loss process, and radial diffusion by magnetic and electric field perturbations. This model used in static conditions gives results which are in a good agreement with those of NASA's AP8 model. However, a better knowledge of thermospheric densities, radial diffusion, and improved measurements of proton flux near synchronous orbit altitudes may be necessary to improve future model results. We prove that an external boundary condition, which plays the part of an injection source, is essential to radiation belt existence and explains the effect of solar activity and magnetotail dynamics on radiation belt dynamics. ¿ American Geophysical Union 1995