The authors report on the use of US in the evaluation of osteogenesis in the fractures treated with external fixation. From April 1991 to October 1992, fifty patients with diaphyseal fractures were submitted to real-time US examinations, with 7.5-5 MHz probes, mainly on longitudinal scans. The evolutive stage of bone callus was studied and, in the presence of periosteal bone bridge, its length and height were measured. In all but 4 patients US, unlike radiology, allowed the signs of osteogenesis to be detected as early as from day 10 on and the peculiar bilobate pattern known as primary callus response was clearly demonstrated. This stage is of fundamental importance because it allows, when periosteal callus is absent, direct intervention on the dynamic process, by varying the external fixation-bone complex so as to make it more flexible. In 80% of patients periosteal apposition was observed on day 24. Delayed osteogenesis depended on the patient's age and on fracture type. Such pathologic signs as hyperstimuli, pseudoarthrosis and/or hematoma persistence were quite unfrequent findings. Even though the study of US semiology is still in progress, our results are in substantial agreement with literature data and emphasize the role of US in the early evaluation of the bone callus and its evolution.