Background: Tumour hypoxia is an adverse prognostic factor in head and neck squamous cell carcinoma (HNSCC). We assessed whether patients with hypoxic HNSCC benefited from the addition of nimorazole to definitive radiotherapy (RT). Methods: NIMRAD was a phase III, multi-centre, placebo-controlled, double-blind trial in patients with HNSCC unsuitable for concurrent platinum chemotherapy or cetuximab with definitive RT (NCT01950689). Patients were randomized 1:1 to receive nimorazole (1.2 g/m² daily, prior to RT) + RT (65 Gy in 30 fractions over 6 weeks) or placebo (taken via the same schedule) + RT. The primary endpoint was loco-regional control (freedom from loco-regional progression, FFLRP) in patients with hypoxic tumours, defined as greater than or equal to the median hypoxia score of the first 50 patients analysed (≥0.079), using a validated 26-gene signature. The planned sample size was 340 patients allowing for signature generation in 85%, assumed HR 0.50 for nimorazole effectiveness in the hypoxic group, and requiring 66 loco-regional failures to have 80% power in a two-tail log-rank test at the 5% significance level. Results: 338 patients were randomised by 19 UK centres from May 2014 to May 2019, with a median follow-up of 3.1 years (95%CI 2.9-3.4). Hypoxia scores were available for 286 (85%). The median patient age was 73 years (range 44-88); and clinical factors were balanced between the arms, both for the whole population and hypoxic group (Table). There were 36 (25.9%) loco-regional failures in the hypoxic group, where nimorazole + RT did not improve FFLRP (adjusted HR 0.72; 95% CI 0.36-1.44; p=0.35), or overall survival (adjusted HR 0.96; 0.53-1.72; p=0.88) compared with placebo + RT. Similarly, nimorazole + RT did not improve FFLRP or OS in the whole population. In total (n=338), 73% of patients allocated nimorazole adhered to the drug for ≥50% of RT fractions. Nimorazole + RT caused more acute nausea compared with placebo + RT (CTCAE v4.0 G1+2: 56.6% vs 42.4%, G3: 10.1% vs 5.3%, respectively; p=0.003), with no differences in other early or late toxicities. Conclusions: Addition of the hypoxia modifier nimorazole to RT for locally advanced HNSCC in older and less fit patients did not improve loco-regional control or survival. Clinical trial information: NCT01950689.