The classic Ortolani and Barlow signs are routinely used to diagnose hip instability secondary to severe acetabular dysplasia in the newborn. However, eliciting a positive sign depends largely on the experience of the examiner and the subjective amount of manual pressure the examiner applies on the baby's hips. Furthermore, these signs do not give a clue for the selection of a maturation or immobilization device after reduction of an unstable hip: below-knee hip spica, above-knee hip spica or a Pavlik harness. The aim of this study is to describe a clinical sign that could be useful in detecting hip instability of the newborn and to decide the proper treatment in a more objective manner: knee extension provokes dislocation of the ipsilateral unstable hip.