Case summaries and histological analyses of eight cases of limb lengthening by callus distraction are reported. Problems developing after lengthening required excision of some of the lengthened bone. Specimens were harvested and analyzed from 102 days to 4 years after lengthening. The earliest bone laid down was woven bone with high surface cellular activity. This soon became overlaid with lamellar bone, particularly at the cortex. Later Haversian architecture of cortical bone reappeared, and separate medullary bone developed, progressing toward the normal pattern, in contradistinction to the very abnormal bone that appears to be laid down subsequent to the Wagner method of lengthening. Of particular value in this study were the relatively large sizes of the specimens, allowing truly representative information on collagen orientation and bone surface architecture. The near-normal appearance of the new bone formed by callotasis gives confidence to surgeons about to embark on limb lengthening.