Since the diagnosis of rib fractures caused by blunt thoracic trauma is often missed on X-ray, attempt have been made to detect unknown fractures with ultrasound. Ultrasound was initially researched, as a diagnostic technique in a preliminary study of 10 patients with known rib fractures. Subsequently, in the main study, 21 patients with clinically suspected rib fractures but normal thoracic X-rays were examined with ultrasound, and rib fractures were detected in 16 patients. The diagnosis of rib fractures by ultrasound was substantiated by an increase of activity in the technetium scintigram in 11 of 11 patients. We found further increases of activity in the scintigram in two more ribs in 7 of these 11 patients. This finding is explained by the presence of nondislocated rib fractures that are veiled to ultrasound when a disconnection of the rib's contour is missed during respiration. The diagnostic procedure is explained with reference to case studies. In summary, findings recorded in this study showed that ultrasound investigation is a more reliable method of diagnosis than X-rays examination.