We present first-principles calculations of hyperfine fields of heavy impurities in BCC Fe. In particular, the effect of lattice relaxations on the calculated hyperfine fields are studied. The calculations are based on a full-potential Korringa–Kohn–Rostoker Green's function method for defects and employ the local spin–density approximation for the exchange and correlation effects. The non-spherical parts of the potential and the charge density are treated correctly, while the forces are calculated by an ionic version of the Hellmann–Feynman theorem. Lattice statics methods are used to describe the longer ranged relaxations. The calculated hyperfine fields of 5sp and 6sp elements are compared with the available experimental data and it is shown that the inclusion of lattice relaxations in the calculation improves the agreement with experiments.