The aim of the study described here was to evaluate experience in the diagnosis of a subperiosteal abscess secondary to mastoiditis by means of ultrasound. Ten consecutive cases in which an ultrasound examination of the retro-auricular region was performed for suspected mastoiditis were identified. In nine cases, we found a poorly demarcated, inhomogeneous, irregular, poorly perfused lesion abutting the outer table of the cranial vault, with elevation of the outer periosteum and a clearly delineated defect of the cortical layer. In one case, there was additional invasion of the insertion of the sternocleidomastoid muscle on the mastoid process of the temporal bone, raising the suspicion of Bezold mastoiditis. Ultrasound may help in selecting patients for further imaging and might spare computer tomography, especially in sensitive patient groups such as children and pregnant women, if a defect of the outer cortex of the temporal bone can be excluded with certainty.