The long term results are reported of a trial involving 120 patients with chronic multiple sclerosis who were randomised to receive either 100% oxygen at 2 atmospheres absolute (ATA) for 90 minutes daily for 20 sessions or placebo therapy with air using a simulated compression procedure. The previous finding of subjective improvement in bowel/bladder function at the end of treatment was not confirmed by objective urodynamic assessment. The treatment did not alter disease progression as measured by the Kurtzke disability status scale nor did it alter the rate of acute relapse. There was less deterioration in cerebellar function at one year in the treated patients as measured by the Kurtzke functional systems scale. No other differences were found between the two groups. Psychometric tests and measurements of lymphocyte sub-populations showed no treatment related effects. Evoked potential studies showed no improvements but there was a significant reduction in amplitude of the visual evoked potential in the treated patients at the end of therapy. This might indicate a reversible degree of retinal damage induced by oxygen toxicity.