Adjacent segment disease (ASD) is a common complication and cause of reoperation following lumbar spinal fusion. Although most commonly performed with titanium or other metal rods, the use of polyetheretherketone (PEEK) rods in spinal fusion has increased. It has been hypothesized that the material properties of PEEK, compared to titanium, allow for less strain on, and therefore less degeneration of, segments adjacent to the fused segments. A systematic search of PubMed and ScienceDirect returned 967 abstracts, of which 13 full-text articles were included in the PEEK rod group and 12 articles were included in the titanium rod group. The two cohorts had similar sex distributions, although the PEEK rod group was significantly younger (61.1 years vs. 62.5 years, p < 0.0001). A meta-analysis using a random effects model with restricted maximum likelihood estimation resulted in an overall incidence rate of 0.026 cases per person-year in the PEEK rod group (95% confidence interval (CI) = 0.010-0.043) and an overall incidence rate of 0.024 cases per person-year in the titanium rod group (95% CI = 0.014-0.033). One-way analysis of variance revealed no significant difference in the incidence rates of ASD between the two groups (p = 0.8422). In conclusion, there is no evidence of any difference in the rate of ASD when performing lumbar fusions with PEEK rods versus titanium rods.