The accuracy of panoramic radiography (RX) and ultrasound (US) in the evaluation of both the length of the osteotomic gap and the quality of new bone formation in patients undergoing mandibular distraction osteogenesis (DO) was assessed, verifying the results against intraoperative and histologic findings. In 31 patients, three RX and three US examinations were performed after DO. RX and US findings were each independently compared, at the time of distractor removal, to the direct intraoperative measurement of the osteotomic gap and to the histologic evaluation of the maturity of a resected specimen. No significant differences (P>0.1) resulted at any step between RX (average length: 18.19 mm) and US (18.29 mm) measurement of the osteotomic gap. In the assessment of the callus maturity the difference between RX and US (P<0.001) was statistically significant: at the final control the maturity score averaged 0.612 at RX, 3 at US and 3 at histology. RX and US are equally reliable in the measurement of the osteotomic gap, while US is much more accurate than RX in the evaluation of the callus maturity. US-based follow-up might allow a safe shortening of the fixation period.