A case is presented of an infant with Erb's palsy whose posterior disl
ocation of the humeral head was identified using high-resolution ultra
sound following non-diagnostic plain radiographs. Posterior dislocatio
n is associated with Erb's palsy but the diagnosis can be delayed. We
suggest the early use of ultrasound in patients with Erb's palsy, as t
his technique is a safe, quick and reliable method of excluding poster
ior dislocation.