Callotasis is a new technique of limb lengthening involving slow distraction of the callus formed in response to a proximal submetaphyseal corticotomy. Using a dynamic axial fixator with telescoping capabilities, distraction begins after 2 weeks. When the required length is attained, the fixator is held in the rigid mode until radiographic evidence of callus is observed. The locking screw is then released, and dynamic axial loading is instituted to promote corticalization. One hundred bony segments have been lengthened; 50 patients had limb length inequality, and 23 had achondroplasia. The mean lengthening achieved was 22% (maximum, 58%). There were 14 complications (14%).