In The Lancet Oncology, Nicklas Oscarsson and colleagues1 present the results of a randomised, controlled trial reporting the effects of hyperbaric oxygen therapy in the treatment of moderately severe radiation-induced cystitis. A Cochrane analysis2 of placebo-controlled trials shows that perception—and not physiological improvement—is at the heart of the placebo effect.2 Another potential criticism is that patients in the control group were allowed to cross over after the primary outcome assessment at 6–8 months after randomisation, and nearly all did so. ...investigators, including myself, have observed that a sizeable proportion of patients treated with hyperbaric oxygen therapy for radiation cystitis will have symptom recurrence even after an apparently adequate course of therapy, but can often be salvaged with additional hyperbaric oxygen treatments.6,7 Long-term follow-up in patients from this study will be essential to establish the durability of response to hyperbaric treatment and the need for additional treatments.