AbstractIn controlled trials using the hyperbaric oxygen chamber in the radiotherapy of carcinoma of the bladder, no benefit was found in a series of 40 patients treated using 30 fractions and a total dose of 6,000 R and in a series of 27 patients treated using 15 fractions and a total dose of 4,725R.In an uncontrolled trial in advanced cases using six fractions and a maximum tissue dose of 3,600 R results seemed improved despite the selection of cases.A new trial comparing this six-fraction technique with multiple fractions in air is now being carried out. Similar trials are being performed in a number of radiotherapy centres and it is planned to pool results so as to obtain a significant result at an early time.In the work reported, doses have been quoted in R units; the absorbed dose can be obtained by multiplying stated values by 0·97.