An exact equation of evolution, with the structure of the diffusion equation, is derived for the phase‐space probability density of a system whose Hamiltonian is random. No assumption need be made about the magnitude of the fluctuations nor about their time scale. In the limit of short correlation time and weak correlations, the equation reduces to the equation of Birmingham, Northrop, and Fa¨lthammar. The reduction of the Fokker‐Planck equation to diffusion form is also demonstrated.