A consistent model for the calculation of fast reactor assembly diffusion coefficients is presented. Allowance is made for the treatment of both sodium-filled and sodium-voided lattices, with the same approximation applied, which is essential for the study of the sodium-voiding effect. The hexagonal steel tube is also taken into account. The formalism is implemented in the ASDIC (Assembly Diffusion Coefficients) program. Numerical results and comparisons with results of other models are given.