BACKGROUND CONTEXT Although fusion rates between posterolateral fusions with pedicle screws (PLFs) and anterior lumbar interbody fusions with pedicle screws (ALIFs+PS) have been well studied, there has been no consensus on the better technique with respect to clinical outcome and nonunion rates. Most studies determine nonunion rates based on radiographic studies; however, many of these nonunions are asymptomatic. A more clinically useful measure is the operative nonunion rate. To date, data comparing operative nonunion rates in PLF and ALIF+PS are limited. PURPOSE To determine whether there is a difference between nonunion reoperation rates in PLFs vs anterior lumbar interbody fusions with pedicle screws (ALIFs+PS). STUDY DESIGN/SETTING Retrospective cohort. PATIENT SAMPLE The cohort comprised 8,340 patients aged ≥50 who underwent primary elective lumbar spinal fusion (L1-S1) for degenerative or deformity related spinal disorders (2009-2018). OUTCOME MEASURES Operative nonunions. METHODS Adult patients (≥18 years old) with degenerative disc disease who underwent primary elective PLFs and ALIFs+PS for 1-level and 2-level fusions (L4-S1) between 2009-2018 were identified using data from the Kaiser Permanente Spine Registry and followed until validated operative nonunion, membership termination, death, or 03/31/2019. Descriptive statistics and two-year incidence rates for validated operative nonunions were calculated by fusion level, fusion type, and levels fused. Time-dependent multivariable Cox-Proportional Hazards regression was used to evaluate operative nonunion rates with adjustment for covariates or risk change estimates more than 10%. RESULTS We identified 2,061 patients (PLF:1491, ALIF+PS:570) with average follow-up time of 4.8(±3.1) years, and average time to operative nonunion 1.3(±1.2) yrs. Comparatively, single and multilevel incidence rates for nonunion were higher in PLF vs ALIF+PS: 1-Level: 0.9% (95% CI=0.4-1.6) vs 0.6% (95% CI=0.1-2.1); 2-Level: 2.0% (95% CI=0.8-4.0) vs 0.9% (95% CI=0.1-3.3). There were no observed differences in risk of operative nonunions in multivariable models comparing ALIF+PS vs PLF (HR=0.3, 95% CI=0.1-1.1), 1-level vs 2-level fusions (HR=1.8, 95% CI=0.8-4.3), or by fusion level (L4-L5: HR=1.0, 95% CI=0.4-2.7; L5-S1: HR=2.0, 95% CI=0.7-5.4). CONCLUSIONS A large cohort of patients with >4 years follow-up found no difference in operative nonunions between PLF and ALIF+PS. Further research is warranted. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.