A minority of individuals have an impaired ability to oxidize sulphur. An increased prevalence of this poor S-oxidation capacity has been observed in patients with established rheumatoid arthritis (RA). The role of this enzyme system in the development of disease was studied by testing the S-oxidation capacity of 54 patients with recent onset symmetrical poly-arthritis, who were followed up at 1 and 4 years. Those patients with persistent disease at 1 year had a prevalence of poor S-oxidation of 69%. At 4 years 74% of those with a diagnosis of RA were poor S-oxidizers compared to 31% of those who were asymptomatic (P less than 0.01). A defective ability to oxidize sulphur appears to predispose to persistent clinical disease.