We wished to assess the true usefulness of ultrasound in limb lengthening by callotasis to ascertain if ultrasound can actually replace radiography at certain stages of the procedure. We examined radiograms and ultrasound scans of 100 segments (50 femurs, 50 tibias) at different stages during the lengthening procedure. Ultrasound proved effective in replacing radiography during the period of distraction to assess ossification of the newly formed bone. Ultrasound moreover gives more— and more detailed—information than radiography in the study of possible problems associated with the soft tissue-cortex-screw interface.