Ideally, the problem of hip dysplasia should be solved by the time the child reaches walking age. In order to achieve an early diagnosis, clinical screening programmes have been implemented worldwide. Although favourable results from neonatal clinical screening have been published, 1,2 others have shown less enthusiasm. 3,4 As was recognized by WynneDavies 5 there are two types of hip dysplasia, one based on neonatal instability, the other based on a congenitally shallow acetabulum. Evidently, clinical neonatal screening, trying to detect instability, will fail in diagnosing the stable, but dysplastic hip. For this latter category, as well as for cases that may develop at some time after birth, imaging studies of the hip joint are indispensable. The emergence of ultrasound as a new imaging technology 6,7 has raised two important questions: (i) How reliable is it compared to radiography?; and (ii) Can it aid in screening newborns? The purpose of this article is to define the present role of ultrasound in the management of hip dysplasia, and to answer these two questions. axis of the child (Fig. 2). Special hammocks may be used, but are not necessary in the author's experience. A linear scanner is superior to a sector scanner; depending on the age of the child a 5 or a 7.5 MHz transducer should be used? One must realize that each ultrasound image consists of one slice of the pelvis as in AP tomography. The whole hip joint should be examined front to back, but the examiner must make an effort to find the most appropriate and reproducible plane for assessment of the joint (the reference or standard plane)? This plane shows the iliac wing as a straight line (Fig. 1). I f a slice more ventrally is chosen, the projected iliac bone curves outward (Fig. 3), and a more dorsal slice shows an inward curvature (Fig. 4). The overall picture, in combination with measurements of the inclination of the bony and cartilaginous acetabulum (Fig. 5) determine the status of the hip joint. Ultrasonography offers a unique possibility to visualize hip joint instability in real-time (Fig. 6), as was also realized by Harcke et a12 As Graf has emphasized, static and dynamic examination do not exclude but rather complement each other?