&NA;In order to determine the extent of cervical spine immobilization provided by the thermoplastic Minerva body jacket (TMBJ) 20 healthy male subjects underwent analysis of cervical spine motion before and after TMBJ placement. Maximal cervical flexion/extension and lateral bending were measured from lateral and anteroposterior roentgenograms, respectively. Maximal cervical rotation was measured from overhead photographs. The TMBJ significantly limited flexion/extension at each level of the cervical spine, as well as rotation and lateral bending (P< 0.001). Flexion/extension at each cervical level was found to be equal to that allowed by the halo with body jacket at most levels and less at the occiput‐C1, C3‐C4, and C6‐C7 (as reported in studies using similar methodology). The present study suggests that the thermoplastic Minerva body jacket is a valuable option for rigid external immobilization of the cervical spine. (Neurosurgery25:363‐368, 1989)