Background: The purpose of this study is to compare 1-year postoperative clinical outcomes between posterior instrumented fusion with (P/TLIF) and without (PLF) interbody fusion in patients with isthmic spondylolisthesis. Methods: This is a multi-centre retrospective study using the Canadian Spine Outcomes and Research Network. Adult patients who received surgical management for isthmic spondylolisthesis were included in this study. The primary outcome was change in Oswestry Disability Index at 1-year. Secondary outcomes were change in EQ-5D, SF-12 PCS, back pain, leg pain, estimated blood loss, length of surgery, length of stay, rate of transfusions and adverse events. Descriptive statistics, Student t-test, Chi-Squared test, and stepwise multivariable regression were used for analysis. Results: A total of 300 patients (252 P/TLIF, 48 PLF) were included in this study. The mean age was 50 years. The P/TLIF group had poorer baseline leg pain scores (t=2.02, p=0.01). There were no significant differences in primary and secondary outcomes between the two groups. Addition of interbody fusion was not a significant variable in the stepwise multivariable regression analysis. Conclusions: There were no significant differences in clinical outcomes at 1 year. Addition of interbody fusion was not associated with higher complication rates or length of stay.