BACKGROUND: The common cold is the most common infectious disease affecting humans and has a substantial economic impact on society. Human rhinoviruses, which cause almost two-thirds of colds, have demonstrated temperature-dependent replication which is optimal between 33 degrees C and 35 degrees C. METHODS: This randomised, single-blind, parallel-group trial completed at a single-centre in New Zealand, recruited 170 participants aged 18-75 years (mean age 27.5 years) who were within 48 hours of common cold symptom onset and had a symptom score (the Modified Jackson Score (MJS)) >/=7 and a negative point-of-care test for influenza. Participants were blinded to the intervention and randomised (1:1) to 5 days of either nasal high flow rhinothermy (rNHF) (100% humidified air delivered at 35 L/min and 41 degrees C for 2 hours daily) (n=85) or \'sham\' rhinothermy (100% humidified air delivered at 10 L/min and 31 degrees C for 10 min daily) (n=85) and completed daily symptom diaries, which included the MJS, for 14 days, to investigate whether rNHF reduced common cold symptom severity and duration compared with \'sham\' rhinothermy. RESULTS: An intention-to-treat superiority analysis included all randomised participants and showed no difference between treatment groups for the primary outcome, the day 4 MJS analysed by analysis of covariance: mean (SD) 6.33 (3.97) for rNHF vs 5.8 (3.15) for \'sham\'; estimated difference (95% CI) 0.37 (-0.69 to 1.42), p=0.49. There was no difference in time until resolution of symptoms: mean (SD) 5.96 (4.47) days for rNHF vs 6.42 (4.09) days for \'sham\'; estimated difference (95% CI) 1.02 (0.75 to 1.38), p=0.91. There were no serious adverse events related to the study treatments. CONCLUSIONS: This well-powered, single-blind randomised controlled trial does not provide evidence that 5 days of rNHF (100% humidified air heated to 41 degrees C delivered at 35 L/min for 2 hours daily) reduces common cold symptom severity or duration. However, investigation of rNHF in the treatment of influenza is warranted. TRIAL REGISTRATION NUMBER: ACTRN12617001340325.