N-Acetyl-beta-D-glucosaminidase (NAG) is a sensitive indicator of renal damage. When the urinary NAG levels of 60 patients with rheumatoid arthritis (RA) were measured a highly significant difference was detected between the mean urinary NAG excretion of the patients with RA (341 +/- 294-92 nmol h-1 mg creatinine-1) and that of 60 matched controls (67-53 +/- 16-93 nmol h-1 mg creatinine-1). In 10 patients studied no correlation could be shown between the urinary NAG levels and levels in either serum or synovial fluid. A significant positive correlation was noted between urinary NAG excretion and disease activity as estimated by the Lansbury activity index, the Ritchie articular index, and the haemaglobin level. An abnormally raised urinary NAG excretion was detected in eight out of 20 previously untreated patients with RA, which suggested that the rheumatoid process may affect renal function.