The distraction gap at eight sites in six patients undergoing limb lengthening was monitored by both standard radiographs and ultrasound. Using the measurements obtained we were able to assess the rate and quality of callus formation at the distraction site, something that radiographs alone do not enable one to do. We suggest that ultrasound has a valuable role to play in guaging the optimum rate of lengthening for a particular patient and commend it as a technique to complement radiographs in the follow-up of the increasing numbers of patients undergoing distraction osteogenesis.