We compared delayed distraction (DD) with immediate distraction (ID) in bone-lengthening. Open femoral diaphyseal osteotomy was performed on 24 three-month-old lambs, and extemal distractor fixators were applied. In the ID group (n 12 ), distraction commenced on the first postoperative day; in the DD group (n 12 ), distraction was delayed until the tenth day alter surgery. In all the animals, the femur was lengthened by 2 cm at the rate of 1 mm/day. The animals were killed 1, 2, and 3 months postoperatively. Radiography and densitometry of the lengthened callus showed that DD, compared with ID, improved the quality of the callus with quicker, denser, and more homogeneous bone formation.