OBJECTIVE: Designing surgical strategies for ossified lesions in the upper cervical spine is challenging owing to the complex anatomic structures. The present study aimed to clarify the prevalence of ossified lesions in the upper cervical spine in patients with ossification in the posterior longitudinal ligament and illustrate the clinical features, radiological findings, and surgical outcomes of this abnormality. METHODS: Demographic and clinical data were collected for all patients preoperatively and included age, gender, body mass index, alcohol and tobacco use, history of diabetes, visual analog score, and Japanese Orthopaedic Association score. The cervical angle, morphology of ossification in the posterior longitudinal ligament, K-line, occupation ratio, space available for the spinal cord, high-intensity zone, and compression ratio of the spinal cord were calculated. The operative approach, technique used, and complications were recorded. RESULTS: A total of 38 patients were enrolled. The upper cervical segment in 23 patients was not surgically addressed. In the group with the upper cervical segment addressed surgically, 10 patients were treated with C3-C6 open-door laminoplasty and C2 partial laminectomy, 1 with C1-C7 laminoplasty, 1 with C2-C7 laminoplasty, 2 with C3-C6 total and C2 partial laminectomy, and 1 with C1-C5 laminectomy and occipitocervical fusion. CONCLUSIONS: The development of ossified lesions in the upper cervical spine has a high incidence. Decompressive surgery for upper cervical spine segments should be recommended for patients with severe narrowing of the spinal canal and a high signal intensity that extends to the upper cervical segment of the spinal cord. We hope that the findings from the present study will aid in clinical decision-making and provide useful information that can be incorporated into future guidelines.