The purpose of this article is to describe a new entity, subacute instability of the cervical spine. It is defined as the development of radiographic evidence of cervical instability within 3 weeks of a cervical spine injury when initial adequate roentgenograms show no bony or soft tissue abnormality. Six patients who conform to this definition are reported. Each was found to have developed neurologic deficit and radiographic evidence of instability of the cervical spine on repeat examination when none was present initially. There were four unilateral facet dislocations (two C5-C6, one C6-C7, one C4-C5), one perched facet (C5-C6), and one extension subluxation (C4-C5). The mechanism of subacute instability is thought to be due to the elastic and plastic deformation of the ligamentous structures and discs of the cervical spine. An algorithm has been developed and is described for evaluation of patients with cervical trauma and initial normal radiographs. By alerting physicians to the entity of subacute instability of the cervical spine, it is hoped that injuries of this nature will be discovered so that appropriate treatment can be rendered before a fixed deformity develops.