Twenty-four patients with rheumatoid arthritis and isolated haematuria were investigated for the cause of their haematuria. In 3 patients local urological disorders were identified, including a pelvicaliceal carcinoma. Renal biopsies were performed on the remaining 21 patients, and the most common abnormality found was a mild mesangial glomerulonephritis (71%). Immunofluorescence and electron microscopy in these patients usually gave normal results. The lesions occurred independently of gold or D-penicillamine therapy and were not associated with impairment of glomerular function.