BACKGROUND: Polyetheretherketone (PEEK) and machined allograft interbody spacers are among devices used as fusion adjuncts in anterior cervical discectomy and fusion (ACDF). Most results are good to excellent but some patients develop pseudarthrosis. We compared the reoperation rates for pseudarthrosis following 1- or 2-level ACDF with PEEK or allograft cages. METHODS: This was a retrospective cohort study. We reviewed patients who underwent 1- or 2-level ACDF. The rate of subsequent surgery for pseudarthrosis was calculated for cases confirmed by computerized tomography. Patient-reported outcomes were collected at post-index surgery follow-up and post-revision ACDF follow-up. Radiographic parameters were assessed at a minimum of 1-year post-op on all patients. RESULTS: Two hundred and nine patients were included: 167 received allograft and 42 received PEEK. Subsidence was demonstrated in 31% of allograft and 29% of PEEK patients. There were no significant differences in clinical outcomes between allograft and PEEK groups. Clinical outcomes were not adversely affected by subsidence. Reoperation for pseudarthrosis was performed in 8% of allograft patients and 14% of PEEK patients (not statistically different). Improvement in patient-reported outcome was significantly better for patients without symptomatic post-operative pseudarthrosis. CONCLUSIONS: Both allograft and PEEK spacers are acceptable options for ACDF surgery. Similar clinical outcomes and rates of radiographic subsidence were found. Subsidence was not a factor in clinical outcomes. Reoperation for pseudarthrosis was associated with poor outcomes. A higher incidence of revision for symptomatic pseudarthrosis occurred in the PEEK group, but this was not statistically significant.