Whiplash injury to the cervical spine is poorly understood. Symptoms often do not correlate to the clinical findings. It has been hypothesized that the long-term clinical symptoms associated with whiplash have their basis in mechanical derangement of the cervical spine caused at the time of trauma. Before such a hypothesis can be proven, one needs to document and quantify the soft tissue injuries of the cervical spine in whiplash. The purpose of the study was to quantify the mechanical changes that occur in the cervical spine specimen as a result of experimental whiplash trauma. Utilizing a whiplash trauma model, injuries to human cadaveric cervical spine specimens (C0-T1 or C0-C7) were produced by increasingly severe traumas. The flexibility tests determined the motion changes at each intervertebral level in response to 1.0 Nm pure flexion-extension moment. Parameters of range of motion (ROM) and neutral zone (NZ) were determined before and after each trauma. Significant flexibility increases first occurred in the lower cervical spine after 4.5-g rear-end (anteriorly directed) acceleration of the T1 vertebra. At this acceleration magnitude, extension ROM and NZ at C5-C6 increased (P < 0.05) by 98% and 160% respectively. There was also a tendency (P < 0. 1) for the extension NZ at C0-C1 and C6-C7 levels to increase after the 6.5-g acceleration by 52% and 241% respectively. There were no such tendencies for the ROM parameter. We have identified the threshold and sites of whiplash injury to the cervical spine. This information should help the clinician make more precise diagnoses in the case of whiplash trauma patients.