Although cryogenic data collection has become the method of choice for macromolecular crystallography, the flash-cooling step can dramatically increase the mosaicity of some crystals. Macromolecular crystal annealing significantly reduces the mosaicity of flash-cooled crystals without affecting molecular structure. The process, which cycles a flash-cooled crystal to ambient temperature and back to cryogenic temperature, is simple, quick and requires no special equipment. The annealing process has been applied to crystals of several different macromolecules grown from different precipitants and using a variety of cryoprotectants. The protocol for macromolecular crystal annealing also has been applied to restore diffraction from flash-cooled crystals that were mishandled during transfer to or from cryogenic storage. These results will be discussed in relation to crystal mosaicity and effects of radiation damage in flash-cooled crystals.