We present an ab initio formulation of the interlayer exchange coupling (IEC) between two magnetic slabs embedded in a non-magnetic spacer wherein the spacer and the magnetic slabs as well as their interfaces may be random. This approach is based on the spin-polarized surface Green function technique within the tight-binding linear muffin-tin orbital method, the Lloyd formulation of the IEC, and the coherent potential approximation using the vertex-cancellation theorem. The periods, amplitudes, and phases are studied in terms of discrete Fourier transformations. Numerical results illustrating the theory are presented.