The dose dependence of the lattice parameter change for neutron‐irradiated copper has been measured in the range (1.0–10.8)×1018neutrons/cm2. The lattice parameter changes were measured directly by using a multiply reflecting monochromator, and an analysis of the effects of misalignment in the geometry has been given. The lattice parameter changes, showing a pronounced saturation with dose, were analyzed using second‐order elasticity theory applied to the case of dislocation loops.