Importance: Better biomarkers are needed for human papillomavirus (HPV)-negative oropharyngeal cancer (OPC) to identify patients at risk of recurrence. Lymphopenia and an elevated ratio of neutrophils to lymphocytes (NLR) have been associated with poor disease outcomes in a number of solid tumors. Objective: To test the hypothesis that postradiotherapy lymphopenia and elevated NLR are associated with poor clinical outcomes. Design, Setting, and Participants: This single-institution retrospective analysis included patients with HPV-negative OPC treated from January 1, 1997, through January 4, 2017. Median follow-up was 37 months (range, 2-197 months). A total of 108 patients with HPV-negative OPC and at least 1 complete blood cell count 2 to 12 months after the start of radiotherapy were included. Data were analyzed from August 26 to September 7, 2017. Interventions: Surgery followed by radiotherapy vs definitive radiotherapy, with or without chemotherapy. Main Outcomes and Measures: Absolute lymphocyte (ALC) and absolute neutrophil (ANC) counts were tested as variables affecting locoregional control, recurrence-free survival, and overall survival. Results: Of a total of 108 patients included in the analysis (87.0% male; mean age, 56 years [range, 35-84 years]), 57 received surgery followed by postoperative radiotherapy and 51 received definitive radiotherapy. During treatment, 67 of 79 patients (84.8%) had grades 3 to 4 lymphopenia and 17 of 79 (21.5%) had grade 4 lymphopenia. The ANC recovered by 6 months after radiotherapy, but ALC remained depressed to 1 year after radiotherapy. Posttreatment lymphopenia and elevated NLR were associated with worse recurrence-free and overall survival. The estimated 3-year LRC in patients with and without grades 3 to 4 lymphopenia at 3 months after radiotherapy start was 73% vs 82% (hazard ratio [HR], 0.58; 95% CI, 0.19-1.8); estimated 3-year recurrence-free survival, 36% vs 63% (HR, 0.45; 95% CI, 0.23-0.87); and estimated 3-year overall survival, 34% vs 64% (HR, 0.45; 95% CI, 0.23-0.88). In multivariable analysis, an association with worse overall survival was found for definitive radiotherapy (HR, 3.3; 95% CI, 1.6-7.1) and grades 3 to 4 lymphopenia (HR, 2.6; 95% CI, 1.3-5.5) at 3 months after radiotherapy. Conclusions and Relevance: Lymphopenia and NLR as early as 3 months after treatment start may serve as biomarkers of clinical outcomes in patients with HPV-negative OPC. These patients may benefit from adjuvant treatment intensification or closer surveillance. © 2019 American Medical Association. All Rights Reserved.