BACKGROUND: Although the American Joint Committee on Cancer TNM classification has been amended to include human papillomavirus (HPV)-related oropharyngeal squamous cell carcinoma (OPSCC) as an independent entity, to the authors' knowledge the optimized de-escalating treatment modality has not been established to date. METHODS: The authors conducted a retrospective, nationwide, observational study in patients with HPV-related OPSCC who were treated from 2011 to 2014 in Japan to determine the best treatment modality. RESULTS: A total of 688 patients who were newly diagnosed with HPV-related OPSCC who were treated with curative intent at 35 institutions and had coherent clinical information and follow-up data available were included in the current study. In patients with T1-T2N0 disease (79 patients), both the 3-year recurrence-free survival and overall survival (OS) rates were 100% in the group treated with radiotherapy (RT) as well as the group receiving concurrent chemoradiotherapy (CCRT). The 3-year OS rates were 94.4% (for patients with T1N0 disease) and 92.9% (for patients with T2N0 disease) among the patients treated with upfront surgery. In patients with stage I to stage II HPV-related OPSCC, the 5-year recurrence-free survival and OS rates were 91.4% and 92%, respectively, in the patients treated with CCRT with relatively high-dose cisplatin (≥160 mg/m2 ; 114 patients) and 74.3% and 69.5%, respectively, in the patients treated with low-dose cisplatin (<160 mg/m2 ; 17 patients). CONCLUSIONS: Despite it being a retrospective observational trial with a lack of information regarding toxicity and morbidity, the results of the current study demonstrated that patients with T1-T2N0 HPV-related OPSCC could be treated with RT alone because of the equivalent outcomes of RT and CCRT, and patients with stage I to stage II HPV-related OPSCC other than those with T1-T2N0 disease could be treated with CCRT with cisplatin at a dose of ≥160 mg/m2 .