OBJECTIVE This study aimed to estimate the risk of spinal cord injury (SCI) in patients with cervical spondylotic myelopathy (CSM) with and without ossification of posterior longitudinal ligament (OPLL). Also, the study compared the incidence rates of SCI in patients who were managed surgically and conservatively. METHODS This retrospective cohort study covering 15 years analyzed the incidence of SCI in patients with CSM. All patients, identified from the National Health Insurance Research Database, were hospitalized with the diagnosis of CSM and followed up during the study period. These patients with CSM were categorized into 4 groups according to whether they had OPLL or not and whether they received surgery or not: 1) surgically managed CSM without OPLL; 2) conservatively managed CSM without OPLL; 3) surgically managed CSM with OPLL; and 4) conservatively managed CSM with OPLL. The incidence rates of subsequent SCI in each group during follow-up were then compared. Kaplan-Meier and Cox regression analyses were performed to compare the risk of SCI between the groups. RESULTS Between January 1, 1999, and December 31, 2013, there were 17,258 patients with CSM who were followed up for 89,003.78 person-years. The overall incidence of SCI in these patients with CSM was 2.022 per 1000 person-years. Patients who had CSM with OPLL and were conservatively managed had the highest incidence of SCI, at 4.11 per 1000 person-years. Patients who had CSM with OPLL and were surgically managed had a lower incidence of SCI, at 3.69 per 1000 person-years. Patients who had CSM without OPLL and were conservatively managed had an even lower incidence of SCI, at 2.41 per 1000 person-years. Patients who had CSM without OPLL and were surgically managed had the lowest incidence of SCI, at 1.31 per 1000 person-years. The Cox regression model demonstrated that SCIs are significantly more likely to happen in male patients and in those with OPLL (HR 2.00 and 2.24, p < 0.001 and p = 0.007, respectively). Surgery could significantly lower the risk for approximately 50% of patients (HR 0.52, p < 0.001). CONCLUSIONS Patients with CSM had an overall incidence rate of SCI at approximately 0.2% per year. Male sex, the coexistence of OPLL, and conservative management are twice as likely to be associated with subsequent SCI. Surgery is therefore suggested for male patients with CSM who also have OPLL.