Background Delayed union or pseudoarthrosis after posterior lumbar interbody fusion (PLIF) is associated with poor outcomes in health-related quality of life. Therefore, it is important to achieve earlier solid fusion for a successful clinical outcome after PLIF. A few authors reported that biomechanical factors may influence spinal fusion rates. The purpose of our retrospective study was to evaluate the independent predictors of delayed osseous union related to intraoperative procedures of PLIF, and to find ways to reduce delayed osseous union. Methods This was a retrospective study of a completed trial. We reviewed 66 elderly patients with osteoporosis after PLIF (all female, mean age 71 years, follow-up period over 6 months). Lumbar computed tomography scans at 2 months postoperatively were examined for the presence of a translucent zone between autograft and endplate (more than 50% of vertebral diameter), and autograft position with bone bridging (anterior, central, or posterior). Osseous union was assessed by using computed tomography 6 months postoperatively. Results Thirty-three patients (50%) showed complete osseous union, while 33 did not. A translucent zone between autograft and endplate two months postoperatively was observed in nine patients (27%) in the union group and in 23 (70%) in the nonunion group (p<0.01). Autograft position with bone bridging two months postoperatively was anterior, central, and posterior in 17 (52%), 30 (91%), and 20 patients (61%) in the union group, and in 12 (36%), 20 (61%), and seven patients (21%) in the nonunion group (p=0.22, p<0.01, and p<0.01), respectively. Multivariate logistic regression analysis showed that the presence of a translucent zone between autograft and endplate (odds ratio, 0.101; 95% confidence interval: 0.026-0.398; p<0.01) and teriparatide administration (odds ratio, 8.810; 95% confidence interval: 2.222-34.936; p<0.01) were independently associated with osseous union after PLIF. Conclusions A translucent zone between autograft and endplate at two months postoperatively independently predicted delayed osseous union within six months after PLIF. Complete osseous union rates were higher in patients with posterior bone bridging two months postoperatively than in those without. These findings apart from preoperative predictors of osseous union might serve as indicators of how intraoperative techniques affects osseous union enhancement.