Urinary glycosaminoglycan (GAG) excretion was measured in 24 patients with active rheumatoid arthritis (RA) before and after treatment with conventional second-line agents. Urinary GAG excretion was also measured in normal controls, patients with osteoarthritis (OA), and patients with acute myocardial infarction (MI). Total GAG excretion was increased in the RA group and fell after second-line therapy (p less than 0.01). More low than high molecular weight GAG was excreted in the active RA group, and this pattern was reversed after treatment. Excretion of total, high and low molecular weight GAG in the OA group did not differ significantly from controls. Total GAG excretion was increased in the MI group when compared with controls (p less than 0.02) and consisted mainly of high molecular weight GAG. The serial measurement of urinary GAG provides a further index of disease activity and may help to monitor response to treatment.