The authors have calculated the positron wavefunctions and annihilation rates in voids in aluminium as functions of void radius using density functional theory with the non-local approximation to the positron-electron correlation potential developed by Jensen and Walker (1988). Their calculations show the gradual evolution of the positron state, as the radius increases, from a state extending over the whole of the defect volume in monovacancies to a state localized at the void surface in large voids. They find that for voids with radii larger than about 5 AA the lifetime is independent of void size.