STUDY DESIGN: A retrospective study. OBJECTIVE: To determine the clinical and imaging characteristics of postoperative C5 palsy after cervical open-door laminoplasty. SUMMARY OF BACKGROUND DATA: Postoperative C5 palsy is a common complication of cervical laminoplasty, and although there are several hypotheses regarding its etiology, the pathogenesis and preventive measures remain unidentified and many controversies remain. METHODS: A total of 141 consecutive patients (100 men and 41 women) who had undergone open-door laminoplasty for cervical myelopathy between 2006 and 2009 were included in this study. Postoperative C5 palsy was defined as deltoid muscle weakness of a grade <3 in manual muscle test. The patients were divided into 2 groups: 1 that had experienced C5 palsy (PG; the palsy group) and the other that had not (NPG; the nonpalsy group). The parameters evaluated were the incidence of C5 palsy, the level at which spinal cord compression appeared preoperatively, the spinal cord level showing a T2 high-signal intensity lesion on magnetic resonance imaging, T2 high-signal intensity lesions newly appearing postoperatively, and the diameters of the C4/5 and C5/6 foramens. RESULTS: The incidence of C5 palsy was 6.4% (9/141 cases). There were no significant differences between the 2 groups in sex, age, diagnosis, operation time, blood loss, level of spinal cord compression, level of T2 high-signal lesion, or new T2 high-signal lesions. The average diameter of the C4/5 foramen was 1.99 mm in the PG and 2.76 mm in the NPG (P<0.005). In contrast, the average diameter of the C5/6 foramen was 2.35 mm in PG and 2.50 mm in the NPG. CONCLUSIONS: There was a significant difference in the diameter of the C4/5 foramen between the PG and the NPG. The results suggest that the main etiology of C5 palsy is impairment of the C5 nerve root induced by preexisting C4/5 foraminal stenosis.