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STUDY DESIGN Retrospective observational study. OBJECTIVE The purpose of this study was to establish a CT classification system of anterolateral spinal ossification and analyze the effects of sagittal spinal parameters on ossification. SUMMARY OF BACKGROUND DATA Patients with Diffuse idiopathic skeletal hyperostosis (DISH) often present with anterolateral ossification of the spine. Few reports consider anterolateral spinal ossification at each spinal level or explore the influence of sagittal alignment. METHOD 120 patients (79 males and 41 females) over 60 years old who underwent whole spinal CT scans from October 2018 to November 2021 were analyzed. Volume rendering technique images were used to assess the degree of anterolateral spinal ossification in each intervertebral space, and a classification system was established. Sagittal parameters such as thoracic kyphosis (TK), lumbar lordosis (LL), cervical lordosis (CL), sacral slope (SS), and thoracolumbar junction angle (TLJ) of the patients were measured. Multi-factor stepwise linear regression analysis and ordered logistic regression analysis were used to study the effects of the sagittal parameters on ossification grades. RESULTS The median age of the study population was 67 years (63-72 IQR). The new classification system classifies the severity of anterolateral spinal ossification in each intervertebral space into grades 0 to 3 with an intra-observer ICC value of 0.909 and inter-observer ICC value of 0.900. Multivariate stepwise linear regression analysis showed that age (OR=1.30, P<0.001), weight (OR=1.23, P=0.013), and CL (OR=1.19, P<0.001) were significantly correlated with total ossification grade. Multi-factor ordered logistic regression analysis showed that there was a statistically significant correlation between CL and the ossification grades of each intervertebral space of T1~T4 (P≤0.005), between TK and each intervertebral space in T4~T12 (P≤0.019), and between TLJ and each intervertebral space in T10~L3 (P≤0.025). LL was significantly and positively correlated with the ossification grades of each intervertebral space in T7~T11 (P≤0.041). LL and SS was significantly negatively correlated with the ossification grades of each intervertebral space in L3~S1 (P≤0.047). CONCLUSION Our new classification scheme demonstrated good accuracy and reliability for the evaluation of the severity of anterolateral spinal ossification. The effect of sagittal parameters on anterolateral spinal ossification was position-specific. The greater the kyphosis or the smaller the lordosis of a spinal region, the more severe the degree of ossification in the corresponding region. LEVEL OF EVIDENCE 3.